The two bolts that hold the engine plate to the base plate should be sufficient. Are you using the engine stop on the chassis and how much slack in the chain do you start out with? Pick a spot have way between your gears and measure the total deflection of the chain. About 20mm show be good. Also rotate the axle 90 degrees and check again. Keep checking for a full rotation of the axle gear. Could your rear gear be damaged or misaligned?
Try reaching out to Burris, either on FB or by phone, (714) 375-3500.
LAD Specialties customer / tony kart / rotax / kt100