I am repairing my 100 for the grandkids, the rear axle pinion gear is chewed up, but luckily I found a used on from an old moto 4 80cc that is interchangeable...I only had to change out the axle hubs, and they are splined, so that was easy. I now need to fill the rear casing with grease, as I cleaned all the old grease out. What should I use for this? It is sealed when assembled, with no way to check it or fill it further. Any help is appreciated.