GOAL

Decompilation is complete: 13 months later, over 400 subroutines, ~20,000 of lines of commented assembler (a rough estimate) and countless late nights.

Pretty much everything is done to a standard I'm satisfied with. But it's important to also consider what has been omitted:

• Sitdown Motor Code. I have only decompiled a small portion of this, as it's not particularly useful for the rewrite. It's also hard to verify without a simulation of the hardware. Having said that, I've decompiled and commented the motor code for both the upright hardware variants. This could be adapted to a simple rumble controller easily.
• Z80 Sound Code. Currently, this isn't needed. The master 68000 CPU triggers the sound samples and sets sample volumes for the engine pitch. Therefore, it's straightforward to work out what is going on without resorting to lower levels. I may decompile this at a later stage out of curiosity.
• Complex Algorithms. There are a minor number of routines which, although commented and broadly understood, could be better explained by someone with a better knowledge of Maths/Physics than myself. In many cases it's obvious that a routine is performing a square root or similar. But there are certain routines that I might have not fully grasped. Nevertheless, I am confident I have done enough to convert these to meaningful C routines that aren't gibberish. 
• I'm sure I'll find and fix mistakes in my comments as I convert the game.

The upcoming enormous challenge is to rewrite the game engine in comprehensible C++. I will be releasing the rewritten source code as I work, so that those interested can follow my progress. I initially expect visible results to be slow, so please be patient. If there are any questions, please post in the comments and I'll happily answer anything sensible.