I once repaired a roadblasters pcb that suffered from the same issue. The game was running, you could kind of see the road and it was twisting and turning a bit in demo mode with rough edges but no sprites. In my case it turned out it had nothing to do with the slags chips. The cause was a faulty AM27S29 bipolar prom chip at 6/7A labeled "136048 1172". Amongst other things it controls the chip enable signals for the sprite eprom banks.
Do you maybe happen to have a logic probe? If so you could check the output pins 12, 13 and 14 of said chip.