I think it should be possible with the right tools. Not easy, but possible.

First, drain the carbs. Then you'll need a Philips No.2 driver mounted on something like a 1/4" ratchet wrench to get in there for the two inner screws. The two outer screws should be removable with a normal screwdriver. One of those flexible mirrors would be handy too.

The yellow stuff sounds like a build up of fuel over time which has evaporated. If you have a dodgy carb float valve then it will  leak fuel (maybe back into the airbox, I can't remember) and that will evap off leaving the yellow oil.

Also, if you have a multimeter you can check the coil resistance while you've got it all apart.