Rebuilding A Pocket Bike Gearbox

I use a $20 generic Chinese pocket bike transmission on my Scooterguy-mounted engine. For my application and how dependable my MB has become, this 5:1 gearbox is worth its weight in gold. Thankfully, they're cheap and in great supply. It's meant to be filled with oil, as I've purchased new ones with dry-stained oil-remnants inside. I tried running oil in the gearbox, but wasn't keen about the oil leaks. Now I use general-purpose grease in it, and the tranny has been running okay on my bike for 10 months. There's been no leaks, but the original bearings have always been noisy. It'd be nice if ABEC skateboard bearings would fit in the gearbox! At the least, I'm praying that the generic 1.375" sealed bearings used on Staton/BMP engine drive kits will fit into the gearbox.

Anyone ever rebuild/modify these gearboxes? It's not easy getting grease into the nooks and crannies, then putting the gearbox halves together.

Has anyone installed zerk grease-fittings on them? Unsure if the grease would push thru the front seal.

Thanks for help and any responses.

Thanks for the tips on seals and bearings.

I use an 11-tooth 8mm T8F sprocket on the gearbox.