I don't think there are too many options other than watching eBay and Craigslist for used pieces. At one time there was a PTO reverser available that also had standard splines. I've heard of them but haven't seen one come up for sale. I have seen a couple of tillers and bush hogs on eBay and Craigslist. The last tiller was offered for a Buy It Now of a little over $400 and it looked decent in the pics. There are also some vendors here who specialize in these tractors. They may have some suggestions. You'd be better off posting those questions in the Mitsubishi/Satoh section.
Someone else may know more, but I think the mid-mount mower was powered from a pulley off the front of the engine. Again, I'm not sure.
I bought my Beaver to maintain and plow a gravel drive and really wasn't concerned with PTO attachments. It does that very well, so I'm pleased with it. Good luck with yours!
John