Dissapointing performance

Exhaust, too much back pressure will kill any scavenging affects/overlap and dilute the cylinder charge because it can't get out. It doesn't matter what engine it is. It will absolutely kill power. The smaller the engine the more pronounced it seems. Worked on a lot of Harleys dyno testing and excessive back pressure can equate to 12-15 horse loss on 80 ci and torque goes down. Close to 15% loss on exhaust alone. That 50hp number that's been thrown out is probably conservative, but not the only issue. Compression numbers would be nice and verify how much timing chain slack you got going on.