Well, Bruce, it is probably true that just like working on old cars, old grills are a whole different animal than just rehabbing your basic Genesis Silver. If you are only wanting to flip this one, you might give pause before you go too far. There is a market, but it is not very big. You might have it on the shelf a long while before selling it at a profit. For your own use, patience and searching - along with overcoming more of the challenges you have already encountered - could eventually yield you a pretty rare and awesome grilling machine.
Broilmaster has consolidated their replacement stainless steel bow-tie burners to just the smaller version that was originally meant only for the “4” grill. You can, however, still find some aftermarket replacements that are the larger “3” size like your cast iron one.
When I can get on my pc instead of this iPad, I will add a couple links for you.