I have the same engine (M25) in my boat, and know that Maine Sail had the same in his Catalina.
I too have an issue with surface rust, and have been putting it off. Here is a pic;
...and here is an older pic showing the rust with the old motor mounts in place;
When I get up the courage to tackle this job, my plan is to disconnect the exhaust and the driveline (at the coupling), then remove the top nuts of the motor mounts. I will then raise the engine using the scissors jack from a Honda Civic (<2" high when collapsed). Once raised, I will also try to support the engine with a chain. My hope is that I will have room to squeeze my hand in between the oil pan, and that pan that supports the motor mounts.
There are 18 bolts and lock washers that hold the pan to the block (the four between the bell housing scare me the most), and then the dipstick assembly bolts onto the pan as well.
The rust was caused by a leaking raw water pump.