I don't know how accessible the starter is on the car but if you can just bridge the input and output,(two large 13mm nuts) with a 50p piece this will spin the starter but not engage it, so engine won't turn.
the old trick sometimes works of just whacking the starter with a hammer etc, this often jars it into life if it is sticky.
It is the case that these do suffer overheating due to close proximity to the exhaust manifold, but if running in, and not ragging it should be ok for this.
The actual starter faults are normally either worn brushes, or laquer peeling off inside surface of the solenoid housing and physically jamming the actuator rod/piston.
A old bit of knackered 100 grit oiled up can be used to clean the inside of the surface to sort this problem.
If it's brushes it's still dead easy to do, but if it's had a hard life might as well get a recon.
As said though check all leads etc, especially earth connections.
tabetha