I use 6x32 hex head machine screws to bolt my engine to the mount. If the mount has flat rails top and bottom I will drill the holes in the mount large enough to allow the screw to pass through the mount (snug but not tight) and then use a nylon locking nut to secure it on the back side. I will also use a 6x32 tap and thread the hole through the mount. I do this when I do not have easy access to the back side of the mount for a wrench or nut driver. If the mount has a tapered bottom and a nut will nut tighten flush against the bottom of the mount I will again use the 6x32 tap. Typically a screw either 3/4" or 1" will be sufficient. Error on the long side of course.

HTH,

pic 1 - Evolution 52 NXmounted on SIGmounts - using nylocks on the bottom
pic 2/3 - YS1.10 and Saito 91 mounted on a Hayes AL60 mount - flat surface mates with engine but bottom is rounded requiring the threads to be taped.
pic 4 - GPgeneric 90/120 mount - flat on the bottom - either tap or thru bolt
pic 5 - who knows - bottom of beams are not flat - tap
pic 6/8 - other mounts