The transmissions of the same vintage years are identical among the bikes, manxr65, so gearbox from a 1982 R65 is the same as a 1982 R80 or 1982 R100.
As Rob said, the clutch assembly and input shaft on the transmissions changed, so if you've got a 1978-1980 R65 gearbox, get a similar year R80 or R100 engine and it will bolt on. If your R65 is 1981-1984, look for same vintage R100/R80 powerplant. The final drives at the rear wheel are different for the bike (and the ratios are matched to the ratio in the speedometers) so you'd either had a lower top end speed (but a strong tendency to wheelie in most all gears) with the R65 final drive, or change the final drive and speedometer (or get your speedometer recalibrated by Palo Alto Speedo).
