I posted to a Ural Forum the other day explaining my problems with my broken head on the Ural. I received a lot of comments, many of which told me to just forget it and buy a new head. BUT one post stop out with an attitude that matches my own as well a one of the best explanations as to making the fix happen. I love it when people are positive and constructive.
First post for me on the site. Not yet a Ural owner. I've recently had a few cast aluminum engine parts and other cast aluminum repairs in my personal shop. So far all have been successfully repaired. Here is what I would probably do if it was my opportunity to be a hero.
Clean the spigot area with a stainless wire brush and acetone. Remove the valves and associated parts. Mask the valve guides and valve seats with masking tape to protect them. Decide on the best method to match the bore and outer diameter of the repaired spigot. It might be using the mill with a boring head but could also be the lathe with a face plate or four jaw chuck. Get the head mounted and indicate on the remaining part of the spigot. Make sure your setup is repeatable. Remove the head and abrasive blast the damaged area. Preheat the head using a propane torch to help remove some of the contaminants in the base metal. Clean and repeat a few times. I always clean with acetone. Since you will be welding on the head do not use brake cleaner as a cleaning agent as combined with the TIG welding it can make phosgene gas which can be deadly with just one small puff inhaled. If needed you might use a carbide burr on the damaged area to help the surface and remove any embedded abrasive material. Once the area is as clean as you can get it I would try to lay a bead or two of weld on the broken stub. This is to get a good metal base and without any other parts in the way it will be the easiest to weld. You might get some blowup or contamination from the base metal. Nothing wrong with grinding out the pourous area or contaminants and welding again. The goal is to get a good base for future welding. Once you have built up the broken spigot edge an 1/8-1/4 inch I would probably machine a flat area where the new spigot part will be. I'd turn a spigot on the lathe, probably use 6061 aluminum as it welds (I've been using 4043 rod with good results) pretty well and I think it would hold up fine for this application. I would make a reverse cut on the new spigot I just made to match the machined area you did on the head/spigot. Once you have a good fit of the new and old sections I would again give all the parts a light abrasive blasting (if needed) and clean a few times with acetone and heat and a stainless wire brush. I'd clamp the new section to the remaining spigot, verify alignment and and fit and then TIG weld the preheated area. Once cool I would remount the head in the repeatable fixture and match the bore and exterior diameter. Final steps would be to clean the head in hot, soapy water with a good scrubbing to make sure all abrasive material is removed and then to reinstall the valves etc.
I know I'm the new guy here so take this for what it is worth to you. I don't see how you have much to lose as the head is currently unuseable. If the repair fails you still have an unuseable head. If you can make a successful repair you have a useable head and the satisfaction of making the repair and maybe learning something along the way.