Was it the screws holding the motor to the mount, or the screws that hold the mount to the firewall?

If it was the ones holding the mount to the firewall there should be blind nuts on the back side of the firewall. Easy enough to knock them out and replace them. Take outthe tank and look at the backside of the firewall. Should be 4 nuts, look almost like washers from the backside. If they are not glued in, tap them out from the front side using a punch or some other object you can strike the broken bolt with. Don't hit them hard, just light taps.