Some things that give trouble on an old plane that's been setting around

The internals of the fuel tank

Plane was stored with something laying on the plane causing a warped surface or something falling causing a hole in the covering or hanger rash

My old Monokote covered planes, the covering held up for at least 35 years but silk, silkspan, & tissue only last 3 to 5 years at most and it gets brittle.

The engine can seize up. Most often it is the throttle barrel but can be the whole engine. Soaking in alcohol, solvent, or WD40 usually take care of this