The background tile data is stored in the SMD-mounted ROM. I don't know which of the PAL is controlling it, though it is the same for all cave games so it should work.
I would look if all the SMD pins are soldered properly. One problem with these mods is that you get modern high-capacity / 16 bit flash chips only as 3.3V devices, so they are operated out of spec in the PGM. The voltage regulator only changes the input power. This may work fine in many cases, though it is something to be aware of.
(AFAIK the square ASIC is a decoy, and missing from newer revisions...)