How do I tap a engine mount hole?
Well, if you are committed to the hobby you should get set up with a tapper. It is just another expense, I know!
I use 6/32 for most of my 40-90 sized planes. Cost for the tap should run about $5 +/-??. A reversing electric drill is a big help but not totally essential over a straight drill. You will need to drill a simple pilot hole first with a smaller sized bit.