An SAE 300 is a very good welder. Later models used a Continental engine. Get your starter checked out and try to see if you can get the original engine running. I'd guess there's an idler, governor and other stuff that might make an engine swap more involved than you think.
What do you mean, you want to get into the oil rig business? You don't just buy a welder and expect to get work on the rigs. Being a 6 cylinder gas engine, that welder will be thirsty if you plan on making a living with it.