A slow crank when hot is a voltage problem. You need 00 or 000 gauge cables with really good clean ends on a six volt system. Thats positive and ground. Make sure there is no paint under the terminals and use star washers for better contact. Check the voltage at starter when cranking slow,that will tell you a lot. Don't try to check amps with out a really good heavy meter!
Bypass the key switch with a wire straight from batt to coil when its hot. This will tell you if the wire harness is good. Checking voltage at the coil is good idea and can help diagnose the problem.
Joe
