The ZX81 build is the interesting half of this. A stock 1K machine is a brutal budget, and a fruit machine needs a surprising amount of state visible at once.
Did you have to cut anything from the Spectrum version to make it fit, or did the BASIC come out close enough on both?
We work on Game Boy cartridges rather than Sinclair hardware, and what always bites us is that the display eats the budget before the game logic gets a look in. We haven't found a way round that yet. Curious whether the ZX81 does the same to you.