That looks like flux residue, possibly from an attempt to reflow some of the pins on that custom IC. On the SPI motherboard it's fairly common for pins on the custom ICs to come loose. Try carefully cleaning the flux off with isopropyl alcohol.
The video RAM that appears to be bad is the alphanumeric and playfield RAM. The RAMs you changed are for the motion object framebuffer.
Try replacing 21M and if that doesn't fix it, 21N. I believe they're socketed.
Ooh, this game is fun...
Rock 'n Rage
Black Panther
Hot Chase
Gang Busters
Hyper Crash
Polynet Warriors
Fighting Bujutsu
Edit: Forgot to add The Powerpuff Girls
As @caius suggested, it was indeed one of the line buffer SRAMs that was bad (see attached). There's multiple layers of sprites; the only tilemap is the text layer. It was easy to identify which SRAM was bad on a 'scope because it was outputting random shite at times when there shouldn't be any...
The SCSI2SD V5.0b (the small red one) works great with Simpsons Bowling as a CD drive substitute. However, I never managed to get it working with the V5.1 SCIS2SD, no matter what I tried.
The flash ROM daughterboard is VERY unreliable in my experience; I've had several which reported bad flash...