Got the version for my kit mostly finished up...
I nuked the 06AB routine completely and wrote new game start code, but I still want to rewrite 06AB to have that screen be part of the attract mode sequence between the game demo and instructions.
Ok... that part's done now...
The old "coined up" screen appears for a few seconds between the game demo and "instructions" mode, and is active (can add credits and it'll change from "insert coin" to "press start button", etc).
The only thing I don't like is that the code handles both the coin1 and coin2 inputs, When you hit 9 credits, it'll disable coin1, but coin 2 stays active, and you can count up to 99 credits, and the original credit routine will dump out the count in BCD.
My new credit display will print garbage if the count goes over 9 (only room for one character in CREDIT #), which seems to be OK given the way the game was originally written, and that coin2 isn't actually hooked up in the wiring harness.
FWIW, for small stuff like this, I just hand assemble the code and patch the roms directly rather than using commercial tools, although this was a bit bigger than a standard "patch" with ~330 bytes modified between text changes, patches to the old code and new routines.
Code:
# Never go to start mode from coin check
## 1539: 28 09 jr z,$1544
## 153B: 3A 20 80 ld a,($8020)
# 1539: 00 nop
# 153A: 00 nop
# 153B: 3A 21 80 ld a,($8021)
rompatch2 1e 539 28 00 00 3a 21 80