HOW TO BREAK DOWN THE GEAR BOX TO GET TO BEARINGS

Here are some possible causes and solutions:

Belt Issues: If the drive belt is broken or loose, the tines won't turn. Check the belt tension and replace the belt if necessary .
Idler Assembly: If the idler assembly is not properly tensioning the drive belt, the tines won't turn. Adjust the idler assembly or replace it if necessary .
Engagement Cable: If the engagement cable is loose or not tensioned properly, it may need to be tightened or replaced altogether .
Transmission Issues: If there is an issue with the transmission, such as a broken gear, the tines won't turn. Stop using the machine immediately and inspect the transmission .
Since you mentioned that your tiller tines were turning fast after replacing the carburetor and then stopped turning after connecting the throttle cable, it's possible that there is an issue with the engagement cable. You may want to check if it's properly tensioned and connected to ensure that it's not causing any issues How To Fix Tines That Will Not Move - Tiller Repair

Something in the wheels drive is jammed preventing them from turning. That is why the turning belt starts to smoke when it is turning but cannot unstick the wheels. Need to unstick them for unit to work.

Two possibilities. Something has jammed the tines.
Pull the spark plug wire off and tuck it away someplace so as not to touch the plug.
Tip the tiller up and examine the tines for obstacle such as a rock. If there is nothing jammed try to turn the tines by hand. If the they turn freely you most likely broke a shear pin that secures the tines to the axle.Clean the axle and tines and examine very closely for the pin.
If sheared ( broken) you will need to drive the broken piece out and replace the pin.
There may also be a problem in the transmission case. You would need to remove the tines and case to open it

then you need to open the gear box and find out why if it has a forward and reverse the the shifting linkage it could be stuck in one gear and adding the other will lock it up as well

Parts for the engine can be purchased from briggs and stratton
The bearings and seal can be purchased from Napa. Dimensions are required for the bearing inner and outer diameters. (Caliper time)
The tines.... I don't know. My experience on the tines is you either find an old Hahn Eclipse Tiller for parts or you find a good craftsman that can make one. I had to make a LH tine using the broken parts, some forge hardened bar stock and a welder. The tine manufacture took much longer than expected...!
Kelly

Well hopefully it's not the transmission, as sometimes these tend to go. Another common but cheaper repair are the two bearings where the tines connect to the unit.

MORE THAN LIKELY.. The bearing is located above the worm gear housing, the part connected to the tines beneath the guard. This removal can be somewhat frustrating but if you remove it and remove the engine flange you'll be able to manually spin the drive shaft and see if it's the worm gear housing/lower bearings or if it's the more likely bearing on the clutch drum or the clutch itself.

Your worm shaft or tine shaft and gear assembly are probably shot. Welcome to the club. Long live American manufacturing.

It is going to be a transmission issue. You can try with the engine off, turning the trans pully by hand in the opposite direction that the motor was turning it.
Some gears in the trans got crossed up and locked.

Without really knowing what is broken it will be tough to give a firm idea of the cost. However, you can head over to www.partstree.com. They have a great detailed parts break down and costs associated with it. Looked up your part numer and can find at this link: http://www.partstree.com/parts/?lc=mtd&mn=210-031-000+%281990%29

the tines are usually connected with shear bolts designed to break away when u hit something as a safety precaution so that u dont break anything in the transmission. its common that people dont know this and replace them with normal bolts. if this was done and the tines are not moving now chances are uve got a snapped gear shaft