KZ 650 head on KZ 750 barrels? both 1980 engines

Okay just trying to use my exsisiting parts up. Will a 650 head work on a 750's barrels? Both engines are 1980's...
The 750 head is damaged ( broken studs and tach drive boss ) it came from a wreck I bought. The 750 engine was covered in oil and the compresion is low on cylinders # 1 & # 2..... I have a 650 head.. But I'm sure it needs valve job.
Will it be worth rebuilding the head? Or should pop for a e-bay 750 head ? I'd rather not do the pig in a poke with the e-bay head.....

As long as their both HYVO cam chain the 650 head is a bolt on swap. It has a slighty smaller
comp.chamber and will bump compression maybe a 1/2 point
but no biggie.
Port's are just a little smaller and with the 750 cam's should make
good mid. range power.

Why is compression low on 1&2?

I'm guessing a blown head gasket... It ran after some tinkering clean plugs..and a some head pipes..carbs drained etc. She hissed at me a lot! Cylinders # 3 & # 4 had about #120 psi, # 2 & # 3 were about the same, # 20- # 30 psi... Guessing the head gasket is blown between 1&2... ? Once the head pipes were off you could see the exhaust valves on those two cylinders look like they are torched. The carb mainfolds were all gooped up with silicone as well.
The right side of the head took a bad hit. Frame colasped into the head. Broke off tach drive boss, and exhaust studs... head pipes crushed, most of the front end was destroyed. I'm going to load the engine up and take it to the car wash... Bike has some haunting karma about it.. Anyway....Hoping to salvage the 750 lower end and bolt up parts from my pile!