I'm changing my T-15 HPI engine of my rush evo, and I wish to change it for an OS .15. Do I need a new engine mount? And, Do I need a new tuned pipe?

PS
I don't wish to wate that much

Most likely, you'll have to obtain a mount to fit the O.S. Max engine and get a new exhaust header. The pipe should work, though, once you have the head. You'll have to contact HPI for full details. It's possible that the car won't accept the O.S. engine.