Have you got the standard RN 140 hp version? is there anything wrong with it other than it's a little under powered? The easiest change would be to have Mermaid rebuild it for 180 or 200 hp. That should give you another 15 years.
You also know that it works well in a P22 rather than say the 165hp Yamaha 4 cyl or one of the inline 6 toyota type engines, as it the revs suit the stern drive and prop options and weight etc.
If you haven't dealt with Mermaid they are a family business with lots of experience of these engines and well worth having a chat with them.
Mermaid Marine Diesel Engines Outboard Motors Parts and Installation