You guys make engine mounting much too difficult and expensive. Picture below of cheap and easy standoffs that are a lot better than metal ones having tiny little feet. If you need a box to stiffen up the firewall because the engine torques too much then simply add 1/8" ply to the outside of the wood standoffs, turning them into a box. I've used them from 28CC up to 115cc so far. Years of use and never a problem. That's 3/4" square poplar available at most hardware stores. Each of those legs is good for a few tons of pressure.

For gas filters just run on down to the local lawn and garden or saw shop and pick up 2 of the filtered clunks in the picture. One is the gas tank and the other in the gas can. They are heavy enough to keep the pick up in the fuel. The felt breaks up air bubbles and picks up every last drop of gas in the tank. yes, they are a bit large and they will fit through the opening of every gas tank I've seen so far.