You've probably got a cobbled together Yanmar YM1600. On the Yanmars, there is a surface on the left side of the transmission case, usually near where the operator's ankle would be. It will have something like "YM1600-032202," which it sounds like yours does. That says you have a YM1600, serial number 1091. The YM1600 is a 16 PTO horsepower machine.
The YM2000 is the next size up tractor, and is one of the most common machines out there. The YM1600, unfortunately, is among the most rare. Very few parts from it interchange with other tractors. There are parts available, but not to the same degree as some other choices.
The Yanmars are excellent quality machines, and if yours works for you, that's as good as it needs to be. Do proper maintenance and it should give decades of service, orphan model or not.