1. If ammeter shows charging you don't have a generator or regulator problem as long as it comes to neutral after a bit.
2. Engine should not stop when you disconnect the battery, or do you have some kind of electronic ignition?
3. I guessing with key and lights off when you connect and disconnect battery you get a spark? If yes, simple way to troubleshoot is disconnect one thing at a time and see if you still get battery connector spark that was not the prob, reconnect and try something else. You say you only changes the engine, so work around that area. Sure you didn't change any ign wiring?
Like to hear your feedback and provide more ideas.
Mike
