The yellow mark is where TDC for piston 1 is. I'm presuming that's the correct one as it looks like there's a mark on both sides of the balancer. Problem is now, I've got it timed to 0 degrees and it falls on it's face when punching the gas from idle. Revs fine if I start it at maybe a grand, but practically dies otherwise.
While searching the forum, I found you're supposed to remove the compression ring from the spark plugs, which I had not. I've ordered some new tube gaskets since I think they're leaking. I'll check the tubes to make sure I didn't damage anything, remove the rings from the spark plugs and check the timing again.