I'd have thought they'd be pretty similar. The gearbox on my B3 really isn't that different to the B6 bar the obvious layout change. The internals are very much alike. Only thing i miss on the B3 is a gear diff.
What else has been a big change in 10-15 years? Big bore shocks? Tyres? All things that you could retro fit if you wanted to. Certainly the gearbox on a B3/4 can handle current brushless motors no problem, apart from the fact you'll wheelie every where on high grip!
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