I wouldn't think that the length of the adaptor being a little bit short wouldn't make a significant difference in your compression reading. That being said, 90lbs is very low. Does it even start with that compression? As for a leak down tester, you could probably cobble one together pretty cheaply using an expanding rubber freeze plug for the exhaust port, a scrap of steel plate big enough to cover the reed cage (intake boot removed), a valve stem in the metal plate, and a foot-pedal style bike pump with a gauge. Not fancy, but would probably get the job done. I'm thinking you're going to end up doing a top end and a pair of crank seals at minimum.
1985 Tri-Z 250
1985 ATC250R