Marvellous achievement guys!
At this point, Plus/4 version now or it never happened :D
I don't think a Plus/4 port would be very attractive. Sure, the Plus/4 has a slightly more computer power than the C64, but no acceleration possibilities like running the game on a SuperCPU, Ultimate64, Turbo Chameleon or Mega65. Therefore you would experience the lower playability of a bare unexpanded C64 on the Plus/4, just a little bit smoother, but no way the smooth gameplay of the C128 or C64 accelerators. That "just a little bit smoother" would need to be balanced against the terrible sound capabilities of the Plus/4... end result might be a worse experience than an bare unexpanded C64.
Plus/4 ports are IMO attractive if you can take advantage of the high colour count of the TED, but for a monochrome game like this one...
Well, none of us is a plus/4 user so i guess we won't comment on what plus/4 users want/like/need. But if anybody is interested in driving he port forward - please get in touch with me. it shouldn't be very complicated. And if one wants to enhance the gfx - feel free. Although memory might be an issue.
After thinking a bit more about it: If I would port it, I would target a cartridge. The TED architecture allows mapping 32KB of cartridge ROM and built-in bankswitching with 2 banks. This means that the speedcode that the C128 version uses, can move to cartridge ROM and be executed directly from ROM.
The combination of speedcode and slightly higher CPU speed (compared to C64) may make the speed acceptable, allthough the Plus/4 still lacks the MMU tricks that you can do on the C128.
Choosing for a cartridge also allows solving the sound problem: The Plus/4 allows sound input from cartridge, so you could add sound capability to the cartridge. Perhaps a low-cost SwinSID or so, C64 users would look down on a SwinSID, but for a Plus/4 game it would be a game changer.