It has custom protection that needs to be read out or possibly decapped before it can be emulated. We have a cart and will try to read it out with a trojan at some point but there is an early portion of the protection code that might not be possible to read out at all so it might not be possible with just a trojan and nobody has decapped a PGM chip successfully or otherwise so far. In fact I haven't seen any arcade chip decapped in years.
A PGM motherboard and cart aren't expensive though. They have since released PGM2 and PGM3.