Generaly a plane with "40" (especialy trainers) in the name means it's desined for a 40 size engine and the fact you say the engine mount looks to big for the engine, i'd say the engine is too small, 40 size engines are about the cheapest on the market so either save the 20 for another day or sell it to help fund a 40,Thunder Tiger GP42 is about the best value in a 40 or go to a 46 either the TT46, GMS47, OS46FX.

good luck and welcome to the hobby