So, I have an M-12 and agree that the dipstick could be in a better place. Not sure it's the right thing but before I launched the boat the first time, I changed the oil including filter. Then, added 1.5 Qt. of oil. After a brief run, I pulled the dipstick and noted the position of the oil on the stick. On my stick there is are two lines and the oil was just at the top one. In any event, that is where I keep it (although I've never had to add any between changes). I change the oil at the beginning and middle of each season. Over time I seem to have developed muscle memory and just blindly aim the stick when putting it back in and haven't had much trouble. I check it before every cold start (a hold over form my days farming in my youth where my diesel experience comes from) which means I check it 4-5 times per week in season.
I did replace the diesel part of my unit last winter. The injector pump was leaking (about $600) and when I checked the compression on one cylinder was borderline and the other was well below spec. I priced a new unit and that looked like $12,000 - $16,000 all in. The local diesel shop was estimating $5000 to rebuild the old motor. I found a guy in Tenn. that was selling Kubota pulls that were supposed to be low hours and within spec for around $1100. After a lot though and realizing that I wasn't sure how long I would own the boat I decided to buy one of his units.
When I got the unit it really looked very good. I disconnected my old unit and pulled it into the cabin. My marina crained it out and set it in their shop. I loaded it in my truck and took it home. Over the winter I tore my old engine down (removed all the 'marine' stuff) and transferred it to the pulled unit. It really wasn't much of a job. I took a lot of care, time, and new gaskets in the process. When I had it all back together, I trucked it back up the the marina and the marina staff put it back in my cabin. I pushed it back into position and hooked it up (replacing all the wiring in the process). At launch she started right up (after a thorough bleeding) and ran just great all season with noticeably easier starts, more power, and all old overheating issues gone. I still have the old engine and am thinking I should rebuild it as my winter project for this year. I was worried about the pulled engine but in the end I couldn't be happier with how everything went at about 1/3 the cost of the next best option.
The point of the story is that the Universal M-12 is a Kubota Z430 engine used in the Kubota G4200 garden tractor and parts can be purchased at your local Kubota dealer for a fraction of the cost of Universal parts. So, you could also just order a new dipstick for your motor from Kubota.
