I've been working on this for a few days now and I think it's pretty much finished, at least in this form. I've had some real fun working out a usable timing scheme that will deliver a decent audio and game clock without relying on a crazy involved self-adjusting timer interrupt programmed to coincide with the VDP's vertical sync bit. Or something.
Also, quite unrelated, I decided that if I was a DJ I'd go by the name of DJ NZ and see if anyone got it.
So have a go - it's quite fun. I love the fact it looks identical to the zx81 version, which is a deliberate kind-of nod to the old days where games were ported to systems quickly without much thought to improving them where possible. It's hugely ironic because it was much harder to keep the zx81 aesthetic than it would have been to make a native einstein version
As I've already said the source is up in github, there are some cobwebby corners but a little documentation might be forthcoming. I'll be delighted to talk people through the code, and it would be great to see this as the start of something like a new series of games or conversions of classics. Just sayin'.