The stage II is/was the first improvement on the Orig. Grubee gearbox, and of all the Grubee gearboxes seems to be the quietest. Like Keith I was/am one of the testers and built using the Honda GXH50 motor with the Grubee III gearbox, mounted in a Schwinn Skyliner, and YES it is Noisey but getting a bit quieter as the miles build up. But I don't think it will ever be as quiet as the Stage II gearbox bike I sold to a friend last year.
As for alternative gearboxes. The new DAX Duplex Chain Drive reduction box might be something to consider. Or perhaps the new Staton 5:1 Reduction box with inside drive. Both of these boxes are direct bolt up to the smaller 4-stroke motors that use the 76mm clutch (35cc Robin/Subaru, Honda GX35) and probably could be adapted to the current SBP 4-stroke Shift Kit.
ocscully