OK... a custom 1.2mm head gasket and two base gaskets later and I have a runner. Rode it at a local club dirt track day on Sunday and was surprised how it went. Took it pretty easy as it needed the piston run in but all seemed good. What is funny is that it has a heavily ported barrel yet the power seems very flat, no big hit anywhere. That said, it was noticeably faster than a YZ250H (which was also being run in) and a couple of MX250s, so it seems to be a decent goer.
All good, it seems!