James,
This is pretty easy but let your conscience be your guide. 1. use the stock motor mount if you want to be legal. 2. Use something else and cheat. Honest, it's that simple.
I will say this, I don't like the rule. I doubt very seriously that you would win any races because of a better engine mount. I'm 99.9% sure I would never prostest anyone because of a engine or transmission mount. That said, I didn't write the rule, I just follow what is written the best I know.