I just picked up this plane and had the same question about engine mounting so I began assembly with it. I answered by installing an OS .70 4 stroke at a quartered inverted angle, about 90 degrees from their recommendation. I assumed that the angle resulted from the unusual firewall installation: the mount has to clear the bolts. I discarded their aluminum mount and went with plastic. I don't anticipate a problem since the OS .70 is heavier than most of their recommended engines.

I also found at the Local hardware store 4mm x70 wing nuts to ease removal and installation of the wings. Everything so far seems solid. Have you had yours up yet?