I can't drive out the hinge pins on my Lewmar hatches, circa 1991

If the hinges are bolted they can be released from the bodywork and door by removing the bolts although on many models accessing the bolts can be a very difficult task.

In the case of welded hinges the door must be removed by removing the hinge pins. There is then a lot of careful grinding and cutting to remove the hinge parts from the door and the bodywork and new hinges welded into place.

In the case of a car regularly maintained, including oiling the hinges, removing the hinge pins with a dedicated service tool is relatively easy but can be extremely difficult without the service tool. Cars that have not received a high level of maintenance throughout it's life hinge pins can be extremely difficult to remove needing the application of heat, lots of hammering and repainting afterwards.

The only door bushings I am aware of are the ones on the hinge pins. Correct? If not, please clarify what you mean.
New pins come with new bushings, and the hinges are welded on so the pins are pretty much the only items replaceable. Not a fun job - because of the door stop springs you will need a spring compressor, and you will also need a special tool to drive out the old pins. However, both are available from a parts store or Amazon.
Just open the door and support it from the bottom, and then go for it, one pin at a time - for the upper pin, compress and remove the small spring, drive out the old pin, drive in the new pin and put the spring back. No spring on the bottom hinge - just replace the pin.

You will need to have another hinge welded on.

Sounds like your hinge pins are wearing out. You can get a hing pin and bushing set either at the dealer or any auto parts store. You'll need a jack and a block of wood, or some rubber matting to protect the door. Open the door and place the jack under the door and raise the jack just enough to support the door. Using a hammer, I use a brass hammer but a steal hammer will work, drive the hinge pins out one at a time. With the hinge pin driven out you'll be able to see the wear in the hinge plate, then you install the bushings required to " realign" the pin holes and then reinstall the hinge pin. Sometimes the holes are badly elongated and on the later models were the hinge is welded I remove the door and build the wear area with a wire welder. The bushing method usually works well for most. Good luck I hope this helps.

I don't think the hinges themselves are replaceable, you may find the hinge welded to the frame.. You can however replace the hinge pins. The pins are pressed in, and driven out with a hinge punch. Make sure you have a helper though, the hatch is very heavy.

The door hinge pins aren't very difficult to replace, the hinge itsself is. The hinge is actually welded to the door, so in order to replace it, you will need to take it apart, cut the welds, and weld a new one on, and paint it again. The hinge pins though are doable by about anyone. First, you will NEED these tools. A medium sized hammer, a brass drift punch, a long flat screwdriver, tire iron, or prybar, and something to rest the door on to keep from scratching it up, and to keep from pulling the wiring out. it should be about the same height as the bottom of the door. You may WANT these tools: door spring compressor, 1 extra person. First, start with the bottom hinge pin. You'll notice that there are a couple of washers around the tapered end of the pins. Ignore these; they're just decoration for all intent purposes. Pry out that spring that you see in next to the hinge with the prying utensil that you chose. Using that hammer and punch, drive out the bottom pin. Next, drive out the top pin, taking care to support the door, because when this one comes out, the door WILL fall. This is where your extra person and the milk crate, or whatever you chose to support the door come in. Be careful when driving the top pin out. The punch and hammer will come DANGEROUSLY close to the windshield, and result in a $190 job for a windshield and hinge pin/bushings, versus just $10 for the hinge pin/bushings. This is where you would repair the oblong holes that your door hinge undoubtedly has. You can get by with using something like Qwik Steel putty to fill in the gaps around the bushings if you want to stay cheap (you may end up having to redo it in a few years, but beats welding a new hinge on and having to redo it in a few years anyways. Or, you can choose to build the area up with a welder, redrill the holes, and go that route. Anyways, after you get your new bushings settled in, whether you used the steel putty route, welding, or just installed a new hinge, you can put the door back up there and the pins back in. I like putting the new ones in so the taper is at the bottom (keep thinking they look like they're gonna fall out the way GM puts them in there, though they never do), and then you have an option. If you want, you can leave the spring out that was in there. It will require a door spring compressor to put back in, but if you don't put it in, it won't do anything other than keep it from moving on its own when it's open. I don't bother with it. Hope that's what you're after, and good luck. It's not a hard job, but it can be a wrestling match putting the pins back in. Lastly, the door is a lot heavier than it looks, just FYI when you get it loose. Feel free to ask any further questions, I keep track of these.

dear friend-it will be ok to weld the hinge

sounds to me like the hinge pins are worn out. These could probably be driven out with a drift and replaced with new.

makes me wonder if it is just the hinge that is bent and maybe the body where it attaches to the van. At the same time I doubt some thing easy will do away with the $2k expense (sorry). Maybe an entire back hatch with hinges from salvage yard (make sure to get same color).

You will have to Mcgyver it....You can cut the pin out but usually anything u put in there wil not last long. Also usually the pin goes and the hinge holes get oblonged making it impossibble too repair. I used to cut the hinge out Cause its welded to the frame on most and weld on a new one....