The arcade is the only source, I have reverse engineered the coin-op and deeply studied the code.
The Z80 code is translated in the HuC6280 code but not in a mechanical way otherwise is too slow and big.
Every piece of code is redesigned to behave exactly as the original but with the speed and compactness of a code written with the HuC6280 in mind.
So the enemies behavior is perfectly reproduced, even the bugs!
In the future I will release the source code so everyone can mod it.