The tops speed is hampered by gearing on your BR and is based around the transmission and the final dive located in the rear end of your shaft drive assembly. That along with a rev limiter.
Putting a 350X motor into your big red is going to take a LOT of work for so little gain. Sure you could shoehorn the motor in there but you will have to throw the swingarm and rear axle assembly over your shoulder and start again. Or convert the chain drive 350X to shaft drive. Both of which will cost you big dollars.
Then you'll be left with an overweight, under suspended, abomination that would have cost almost as much as a whole 350X.
And Yes, your right, gearing on the big red is what is holding top speed back. Not power! You could have an 850 big red with that gearing and not go any faster
My toys 85 KXT250A2, 85 ATC250R, 85 Tri-z 250, 06 LT-R450, 04 YFZ450S, 07 125 typhoon x 3, 06 FPV GT.