Wa 450-1 transmission seal replacement

i have a Wa 450-1 with a transmission leak. The leak is coming from the main seal where the drive shaft that connects the engine bell housing and the tranny. on the tranny side is where the leak is.

lets just say its very tight to get in there.
first question. this is a very old machine with a ton of hours which i want to just keep as a spare. I'm curious if adding some type of stop leak is an option. I'm aware of the issues with the converter being in the oil, but has anyone tried stop...

Wa 450-1 transmission seal replacement

Wa 450-1 transmission seal replacement