The HPI T3.0 should be a direct for the HPI Nitro Racer. The engine case is about the same size as the original 15fe and also has cast in place motor mounts too. The crankshaft is dimensionally the same so you should be able to reuse your current flywheel and clutchbell. The ONLY problem I see and it is a major one is that the stock 15fe that came with the HPI RS4 cars of the late 1990s and early 2000s are rated at .6hp whereas the newer T3.0 is rated 2.2hp! This significant increase in power and torque will cause problems with your stock spur gear.
If you do decide on the T3.0 just to see what it will do you may want to take advantage of HPI's factory upgrade program (only for US customers though) where you send in your old engine and pay $65 to upgrade to the T3.0. Don't confuse the T3.0 with the G3.0 - they are different motors designed for different applications, T for on-road and G for off-road. The G3.0 has an entirely different crankshaft (SG).
Good luck and have fun - R/C is very fun hobby though expensive.
