Got to agree with George, the Continental will always be much easier to get parts for than a Waukasha. That said asking what engine will swap out is a really broad, all but impossible question to answer when your talking about a close coupled application like a compressor.
I say that for one reason, and that is the replacement engine will have to match mounting specs exactly with the other one. So, before you can do anything you need to seperate the engine from the compressor and get measurements on everything to know what SAE housing and flywheel you've got. Click on the link below and it will take you to Davidson Sales tech page. There you should see a link about halfway down the list about determining SAE specs for the housing and flywheel. Once you know what your looking for then you can get online and ask around and/or do some online searching for an engine with the same specs.
I know this wasn't the answer you were looking for, but hopefully it will help. Beyond that, if you need any help, etc getting the broken bolts out holler back and I'll be glad to tell you the best way I've found to get them out without screwing up the threads in the block.