I think I like Bob's method better than mine and definitely use steel. I would avoid Aluminum with only 1/8" grip. 3/16 is ok if you have the tools to tap aluminum without overcutting the threads. But, back to the original question since all of these methods are great ways when you are building....but not retrofitting. I still contend that there is no easy way to switch if you didn't plan for it when you built it.
+ when you are done the muffler and needle valve still don't fit the cowl ...aggh...
Ken
Steel isn't needed, and 1/8" aluminum would be perfectly ok, at least up to .46's. You should countersink flathead (socket head) machine screws from the opposite site and put washers and elastic lock nuts on top of the engine mounting lugs. Then bolt these plates to the maple engine bearers. The late Larry Cunningham devised the "U" shaped version and thus it got named "The New Mexico" mounting plate. I made simpler strips for each side and liked the results.
Other than that, some engines have one pair of holes that are the same, but the other two aren't. ST G.51/Royal 45/Magnum/ASP .40>.53 fall into this category. Drilling an extra pair of front holes in the G.51 bearers makes it swap with the others. If you're not willing to do this, oh, well! I would point out that if you crash hard on pavement, the chances are pretty good that the engine is toast anyway, and the extra pair of holes won't decrease the value any!
Steve