Have you swapped both the injector and delivery valve over with another cylinder to see if the miss changes cylinders?..the injector line could have a restriction too so it wouldn't hurt to sus it out even if it is unlikely.
Assuming that you have, do the injectors have the 2 shot injection phase? If I remember correctly some engines have a small pre-injection fuel delivery to kick start the burn, and some don't. Also you might want to look at which export pump you do have as the manufacturer specifies at least 3 different injector crack pressures for various export engines depending on where in the world they are being sold. From memory (I saw this in an online workshop manual) the recommended crack pressures ranged from 2600 psi, to 3200 psi.
An experienced injection shop will be able to help you with a possible head swap, but they are probably going to charge you an arm and a leg for the information/rebuild because they can. Also, they probably don't have any prior experience with the 4j Isuzu as far as performance mods go, which is complicated by the change in spec by your head/piston swap.
The 4jg2 pump has a bigger head (11 or 12 mil. from memory), I would probably try using the complete pump, but the cam disc which pushes the injection piston (plunger) will probably have a different profile when compared to the 4jb1 pump's cam disc. Possibly the 4jb1 pump body combined with the 4jg2 head might be worth considering.
If you have a spare 4jb pump handy you could attempt to swap the head yourself, it is complicated but not impossible to do, and there are rebuild threads describing how to rebuild the pump on the net if you search for long enough.
Here's a guide, but the poster made a mistake close to the end, so you may have to read it through before attempting to put your pump back together.
http://gnarlodious.com/Vanagon/Bosch_Pump/-RebuildEDIT.... The mistake has been edited out since I last visited this page. This pump is from a different vehicle, but most of the internal pump parts are the same as far as reassembly goes. I'm not sure how one goes about getting the right clearance between the camplate and a different plunger shaft / head, maybe checking the thickness of the small spacer and checking the length of both of the plungers will tell if a different thickness spacer is needed.