Re: Time to give up?
I'd really like to see how the front of the engine was mounted. You weren't using rubber in the mounts, were you?. That frame failure would be consistant with the front mount not being solid enough to keep the engine from vibrating. That is a stress failure caused by something being much too loose and allowed to vibrate against the frame.
My guess would be that the front mount fastener failure contributed to the damage I see to the seat tube. How long did you ride the bike with the broken mount?
Failure of the kit supplied spark plug boot is very common. It should be replaced with an automotive quality part during the initial installation of the engine.
Fuel leaks are easy to diagnose and repair. Really not a big issue but they can be aggravating, no doubt. Sounds like you took care of that.
Your frame could be repaired. It will take a welder with experience in welding thin material. I'd suggest cutting the tube and doubling it with a stub on the inside to add strength. Easier said than done however. A doubler welded to the outside of the tube would work but that will increase the size requiring some modification to the engine case/mount. Too bad you can't hook up with Ocho Ninja. He'd be able to help, I'm sure.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"