I had to have the I-P on my Perkins 4-108 rebuilt. I finally traced the leak back to the throttle shaft, but that entailed a fairly long stretch with a drop light in one hand, a rag in the other, and my head next to the idling engine (hint -- think abour EAR PLUGS before you get all contorted into position so you can eyeball the pump.)
I found that rebuild kits are fairly inexpensive, but elected to let the pros do the work. It wasn't cheap, but it was still a lot cheaper than a rebuilt pump. I lucked out in that there was a good shop not too far away that did the work.
Something else to think about as long as the beast will be out of commission -- injectors. If they've been in there a while, you may as well throw a little love their way as well. Relatively cheap and easy to have them cleaned and put back into A-1 condition before you try out that new pump.
Best of luck,