I know that the transmission side seal can not be tested.
I am wondering about the magneto, Generator side seal.
On most snow mobile engines, while it is running, squirt a little starting fluid in close to the crank shaft seal and if the engine increases in idle, then there is a leaking seal.
The engines air intake has to be isolated to keep from drawing the starting fluid into the engine from that side but just a simple quick burst aimed at the seal should to the trick.
With the problems with the carb on My 125 Hummer that I am having I am wondering if maybe there is a problem with the crank shaft seals.
Might be time to pull the engine and just replace them. They are near 20 years in the machine so maybe they are going bad.