My 650 will rev really high when started or not start at all

Kill switch is in the run position ... ???

You have a bad sensor.

is the check engine light on?

The KM90 is a 4mix engine that has valves. You may need to check the valve clearance, using the clearance tool.

Most likely a fueling issue, Check fuel line for air leaks or it could be a carb problem.

I would suggest a full synthetic oil, next time & stay with 5W-30
That will be better on start up

I have no idea why it won't start after only changing the oil

I think that was just a coincidence

I just installed a new throttle body ,because the motor has
acted up for 7 months & unknown the battery was failing
as I was diagnosing some odd behaveour

I was using professional scan tool software & when that
went weird I saw the computer module & battery voltages
going down,key on,engine off. So strange things happen
with todays electronics. When your think your on your game
things go to the twylight zone

Either a short or the circuit is now drawing too much amperage. Disconnect the new indicators and see if that is the problem. Hope this helps.

May need to replace temprature sending unit. Car doesn't know it'swarm.

Check fuse #4 in your fuse panel 

Good Luck!