1970 US90 rebuild and buying parts online

[jsf600rr]

I have begun a complete rebuild of my 1970 us90 engine. It smoked a bit and was spitting oil from the exhaust. So i am cruzing for parts on ebay and i am having a hard time finding a complete gasket and seal kit for the 1970 machine. Now there are tons of late 70's kits for the ATC90. My question is is there a differnce in the engines when the switched over from us90 to atc90 in the 70's?

I am not deep enough into the build to see what was causing it to smoke/spit oil but i believe i have a burnt valve and carbon built up on the piston, and the spark plug was oil soaked. The compression was incredible if you pulled it hard in the wrong part of the stroke it will tear that pull cord handle out of your hand like a bullet. I am going slowly and and documenting with photos as i go. As i have plans for some powdercoating/new paint during the re-build of the machine.

The indications that i have gotten so far seem to suggest that the top end is inter-changeable though out most us90 and atc90 years, but the bottom ends had different versions for 1970-1973, 1974-1980, 1981-on. You guys know all! Help me set my parameters for searching for parts!

[ctbale]

all 90s are the same as far as I know, except for the head gasket, pre 74? had a different style. I reuse copper gaskets though, get em red hot, quench in water, then scotchbrite. I did a 71 atc90 about 9 mos ago, pretty much did the full monty, lapped in new valves, piston/bore, split the case and needed one bearing, timing chain, points/felt,plug. 3 cyl dowels, two 10mx14m and one 10mx20m. never do condensers, The motor is in a beater 78 frame and I beat the hell out if it daily. I just ordered a full vesrah gasket kit and the 6 seal kit, plus one for the gear indicator shaft (shift drum) if you want. same motor as far as I know. dratv, ebay and bikerchickltd