I'm no YS expert but if it were mine, this is what I'd do. If in fact he in fact he " oiled the engine regularly" I would pull the backplate to see what's going on inside. Also pull the glow plug and roll the engine over to clear any excess oil in the combustion chamber. It's possible there is an excess of oil in the motor. It might also be prudent to pull the fuel pressure regulator just to have a look see. It the spare is in fact new and never run, then nothing should be or need be done to that engine. If all looks OK, no rusty bearings or other funk in the engine then go ahead and fire it up and see what you have.

Ken