Actually I think you will find the original /5 front has more travell than the original R80 G/S. The steering head is the same as the G/S and GS models IIRC.
As far as rear wheel travel .. well the G/S has little trouble with the U joint, it is the GS paralevers that give trouble. So you maybe able to get some more here without redoing the drive shaft and/or joints.
I'd have to look at the specks, but I think they came with a 180 watt alternator. 18 inch rear wheel and a 19 front, you should be able to get rear tyres ... the front may be more restricted. Things to look for would be the rear wheel spline between the wheel and the bevel drive - they ware.