tolerances of a hydraulic pump are very tight---even if you do manage to get the housing replicated within a good level of accuracy to the old one, they are still using a damaged and worn housing as a reference point and i'm assuming the other worn parts are being reused in this housing. i would not be at all comfortable in warranting or guaranteeing this sort of repair. plus we still have not addressed the root cause of the housing failure.
my take is that you are opening a can of worms by trying to replicate the housing because if it develops any sort of issue at all within the period of your company's guarantee, even if it relates to one of the reused parts, you are left holding the bag on the cost of replacement, and even outside the guarantee period a very dissatisfied customer who may consider switching service providers.
if they can't stomach 1300 for a new pump, try sourcing a used one without guarantee. serial number? Also discuss trading the forklift in for a used replacement. If you work for a dealer, this might be an opportunity to talk about financing a machine or even leasing a machine with a maintenance contract. There are companies out there that will part out forklifts if it's a mom and pop client.

justinm,
you were exactly right.
upon more thorough inspection I found a hairline crack in the first stage housing.
the crack went through the gear cavity to one of the bolt holes, which is why the oil was leaking out of all plates simultaneously. we are currently exploring having the cracked housing (which is aluminium) replicated via CNC- because the pump, new, runs about $1300...
Thanks for your reply.

I'd say it has a hairline crack in it. When you go to the extreme (deadhead) limits of a cylinder, the fluid pressure builds up in the pump.

Old age, overloading, customer pushing things, or banging into things with the mast. Get it replace. I never rebuild major parts like that myself. Not cause I can't but I''d rather have one that was tested and has someone else's warranty! lol