Steve,
I've never owned a 125, but a nice running 149 is the cat's meow with the hydraulic lift. Got any pics handy? As you stated, the drive train is easier to work on due to the tunnel cover being removable. The hydros in the cubs are bullet proof unless one tow's 'em around without releasing the pressure valves. If it just quit, I would venture to guess it sheared a drive shaft pin or maybe just low on hytran. As far as getting a cub running after sitting, you are on the right track. New gas, check or better yet change the fluids (not feasable if you want to get it running before purchase). Might have to clean the points to get spark. The carb probably is pretty gummed up if it sat with old gas in it. Pop the tunnel cover off and roll the engine over by hand or with the key and see if the drive shaft turns the hydro. Also pull the plug and turn it over checking for compression. If is seems to be in decent shape, looks complete, and you want it, low ball an offer and take it home with you
. If you get it running, the price may double
.
Good luck and keep us posted!