A35.... Mostly widen the exhaust a little and take out the hump they have in it for a smoother flow. Instant gains.
I think that with some good port work, one of their good old 26mm straight pipe exhausts, and some better top speed gearing you could potentially get to 50 even with a 14/12 Sha. It would need that stubby intake tho to get max fuel flow to it. I had a bike doing mid 40's with a similar setup.... Just a biturbo pipe upgrade.