I'm happy to do work on it. There are some weird caveats with this platform so I'm not sure it'll be doable. No sprites and no Collison registers, for example.
I've documented some of it here: https://jasonoakley.com/foxglovz-dev/foxglovz-dev-diary-part-1/
Please let me know if that will prevent it being used with your engine.
There are no hardware sprites on the Spectrum either, the engine XORs sprites and objects on to the screen, then XORs them again to remove them. It does the same for the PLOT command so you might need to re-think these routines. All collisions are coordinate based so it should just be a case of modding the collision routines to work with half the resolution. Spectrum sprites are moved 2 pixels at a time horizontally so there are 4 pre-shifted copies of each frame on that format, you'd have to figure out what would work best for VTech. Objects aren't animated so those are shifted into position at run-time on the Spectrum.
The speaker looks very familiar so no worries there. :)
