What do I do if I have a side exaust engine and want to get a Rear exaust engine do I have too change anything?

You will need a new header. If you use a one piece pipe, you'll need a new pipe too. You will also need a starter box to start the car. Optional things will be lower engine mounts and new smaller flywheel for non pull start. I'm assuming that your side exhaust engine is a pull start model.

Yeah Its the Ofna CD3 Pro RTR and I want too change out the engine too a O.S. .12TZ or STS D3R Factory Modified.

most likely you would need a new flywheel and depending on the shaft of the engine, you need the appropriate clutch. If the engine is an SG shaft engine, make sure you have a centax(push) type clutch. if the engine is a standard shaft, make sure you are using a shoe type clutch, 2 or 3 shoe doesnt matter.

you'll only need hpi header for nitro rs4 3, $12 at tower hobbies. any engine with sg shaft and this header will fit, even the tz 18

Thx