Early models often needed a rebore and crank seals at 15k, plus gearbox hassles, later ones could run for twice that. Chassis went off with age and the (poorly) drum braked model usually ended up with bent forks. Some suffered gearbox faults, seizures and big-end failures. GP125 engine will fit in the 100. Later bikes reliable.
GP100: '80-93, 98cc t/s single, 11hp, 70mph, 80mpg, 195lb GP125: '80-89, 124cc t/s single, 11/15hp, 70/80mph, 65mpg,