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.