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.