I would take the starter out and bench test the motor. I would ground the motor body and hook a jumper wire (I use jumper cables) to the starter motor itself (the small copper bar coming out of the non-drive end of the motort housing). If it spins good I would install a new starter solenoid. If it doesn't spin or sounds slow and weak, replace it. If you have any old starters laying around I'd steal a solinoid off of them to get by until you fix the block. You also may be able to take it to most major parts stores to have it tested as well.