Check Gunbroker, there's usually at least one Century made 80% FAL receiver for sale. I've heard they're almost impossible to finish, but I've never had any experience with them.
Here's one: FAL receiver casting 80% : Parts Kits at GunBroker.com
Also, if you're gonna build an HK91, might want to make it unique:
https://www.apexgunparts.com/product...td4u6movljfj97 Apparently it's a rare, British made kit with a demilled barrel. The barrel just has holes drilled in it so I think you could possibly weld it up and reheat treat it, all for the sake of keeping originality and saving a buck or two, but you'll have to worry about where else to put your 922r parts