I have a RS4 RTR 3 Evo.
I killed my T15 engine (not a bad engine) by hydrolocking it with ARO.
Then I put a OS 18CV-RX on it: fits right in (with Wolfpack Radicals adjustable engine mounts) and I can still use the Rotostart (I replaced the CV-RX pullstarter with rotostart backplate).
I also put a THS pipe on it and geared it as high as possible.
This engine has plenty of power for this car.
Some pics of my nude HPI and one with (non-HPI) body and aluminium/rubber wheels.