The placard on your engine should tell you the exact model.
It might help if you post exactly what is wrong with your engine and where your boat is located. It might help folks recommend where to go for service on the bay. Who knows you may not need a new engine after all.
The Tech Link Stu posted is a great source of info on these engines. A lot of folks here will recommend digging into a rebuild yourself. You may or may not be comfortable doing that.
The most common source for a replacement if you're set on a swap is probably Beta Marine (Beta Marine US Ltd. Distributors for Kubota based marine
). A search of this forum will turn up threads from folks who've installed their engines. They also have quite a few dealers on the Chesapeake.
Last fall/winter Catalina had a number of brand new M25XP's still in crates that they were selling off. I'm not sure about pricing, but an email to Catalina will get you the information if they still have them.
Best of luck. Contemplating a new engine is no fun.