I am building a Sig 1/4 scale cub using a Zenoah 26,has anybody modified the engine mount plate to fit the firewall,since it extends above it,if so,any advice-thanks

Sure, it is done all the time including not even using the mounting plate.

The plate costs $12.99 if you decide you want to go back to original

Just cut off the top corners and drill new holes. no problems