I would have to highly recommend the the RB S7. This is an excellent motor. I may be bias but thats because I have been running rbs exclusively now for he last 7 yrs without any major failures or problems. Rb are easy to tune and hold there tune very well and last for a very long time. One word of warning though they are very tight when new. Take extra care when breaking them(preheat before starting) in and they last.

The S7 will be able to match any other engine in top end. Yes there may be better engines out there but it will hold its own andwont be too far off they're heels. If anything it does lack a little bottom end, but this is easy to fix with the right clutch setup(lighter shoes/heavier springs). Although this is a good thing sometimes as a tame bottom end makes it easier to drive and get the power to the ground.

Yes its true the RB S series are made by novarossi but that doesnt mean they are a direct copy. RB is still in control of all the details of the engine novarossi just manufactures them to the specs rb give them.

Anyone who can win offroad world championships must know what they are doing.

I would get the RB pipe aswell, you could try and find a bundled deal and buy the engine and pipe together.