Reading through this I had alarm bells going off in my head. I had a similar situation rebuilding a very early Super Rat. After SO much work, I was installing the head only to have the very same problem; one of the studs stripped. After walking away from it that evening, I decided, (during a sleepless night), to take the motor in and have timeserts installed, (timeserts are much better than helicoils, IMHO).
I *assumed* 8mm studs. I told the Machine shop to use 8mm timeserts. Imagine my horror when I try to install the studs and they were now too small! So, I purchased 4 3/8" bolts, cut the heads off and tapped them with a plan to tap the head nuts too. Nope.. Now the Cylinder would not go on, the studs were too big. Now I'm in a pickle.
Luckily I had a friend who works in a machine shop and after a quick conference with him, he put them on a lathe and returned 4 cylinder studs, pictured, to me the next day. 8mm on the bottom, 7mm on the top
Everything assembled perfectly after that. What I really liked was that the "shoulder" on the Cylinder studs actually locked the cylinder centered into place very nicely.
The Alarm in my head was that I suspect there may be Ace100 engines out there that have 8mm studs, but all of them I've come across had 7mm studs. Please measure your studs before installing timeserts or helicoils.
Lastly, I would encourage you to have timeserts installed. There was just enough room for an 8mm timesert so there should be enough for a 7mm. I did not disassemble the engine, but I did stuff the crankcase with paper towels and taped over the top and punched out the stud holes. I pulled the engine that morning, dropped it off on the way to work, and had it back in and fully assembled that evening. I think you may still be able to get on the trails before it gets too cold.
$0.02
Roger
Victor