Wicked,

Great looking example of a Hog Bipe. I am very familar with the construction of the cowl and engine mount on this plane. Sadly that Hog Bipe is departed but still my favorite plane. I used glass filled resin type of engine mount. I use #6 socket head sheet metal screws to mount the engine. They will easily self tap into the proper sized hole.

If you insist on using a nut and bolt consider this: To acess muffler bolts through a similar built up cowl on an inverted engine, after model was covered, I drilled a small hole directly opposite of a socket head screw. Through the hole I could insert a hex key to turn the screw. You could do the same by reversing the normal orientation of the screw and nut. Put the nut on "top" of the engine where you can get a hold of it through the cowl opening. The four holes to access a socket head screw would be on the left side of the cowl.

Thanks for the implied compliment

Scott