Well, that's weird, considering sub-78 hitachis don't do AP25/XGD3 at all.
As a general rule, spoof=fail. With sub-78 hitachis, spoofing a higher number drive as a lower number drive is supposed to be one of the rare exceptions, but something's not right, because, well, you are unable to play some games..
What firmware did you flash the new drive with? latest iXtreme?
What was knackered about the old drive? If the PCB is ok, then you might just consider swapping the boards, or lasers. Both are very streightforward, and neither require soldering. You might also take a look at doing some maintainence on your laser. Again, this is assuming that "knackered" means "having trouble reading discs" or "motor is dead" and not "board is borked"
Clean the laser lens with a Q-Tip and 90% Isopropyl Alcohol
Pot Tweak - http://bit.ly/hGWaQ4
Or replace the laser. Sammy <--> Phat Hitachi, Benq <--> Phat Liteon works
Hope this helps!