Damaged Warrior, HELP diagnosis on motor

Bought a warrior, not sure what year, saw it running before i got it. The person who put a new crank/piston in it did not put the two short bolts for the head in it, only the 4 long ones. Also, she said the cam chain tensioner had to be real tight for it not to jump. Duh right. Would this damage the bottom end ? It idles but backfires after it warms up, also wont go past idle. It has a rythmic surge of maybe 100 rpms and if i feather the throttle can get about 400 more rpms but thats it. What would be the diagnosis? Ive tried everything except taking piston out.