I didn't realize it's a brand new engine. Have you retorqed bolts, valve lash adjustment. After a few hot, cold cycles these things need to be checked. It only takes1 bolt to be outta torq spec to ruin a good engine, and a fun day.
To break in an engine, the rings need to seat fairly quick. I found that running different rpms, not letting it set at a constant rpm, seat rings pretty fast. Running it at diffent temps, 250, 275, 300, up and down rpm, hot cycles, repeat, repeat, repeat works real well. Let it cool over night, cold cycles, retorq bolts, check valve lash and do it again. It'll take 3 or 4 days till the bolts and valve lash stay put. Then mount it to what ever, and not worry about bolts backing out.
Keep digging on that motor mount, I'm sure you are getting it.