For the CT5 the belts do affect the speed. Just not in a direct way. The pulleys are to be changed when you change the belts. The optional pulley set in the box offers increased top speed at the expense of acceleration. (gears the car taller which more top speed). The pulleys change the overall gear ratio, the belts are just a way of transferring the power like the universals on a shaft driven car.

The stock belts and pulleys gear the car lower ( ie more acceleration). The optional belts in the car will fit the stock pulleys, but they will skip and you will end up with a car that doesn't go...

As for the turbo 2.5, unfortunately that motor is now discontinued. Some motors you can use are the Novarossi CX15 (this is a big block .15). The big block .15s were the motors meant for the original 1/10 scale on road cars, such as the serpents, piccos, and bmts, if anybody else recalls these cars.

You can also pick up the optional CEN Engine mounts meant for standard .12-.18 engines and you could fit an OS .18 TZ rear exhaust engine or any of the other small block .12-.18s for an upgrade.