Changing injectors isn't tough, just be as fanatically clean about it as you can. It's only a little bit harder than changing spark plugs, and mostly because you will need to bleed the fuel lines after the replacement.
I have never done it on a Yanmar, but a I have a story that might have relevance...
On my old boat I had a 40 year old BMC diesel with standard Bosch injectors. The service manual described in detail how to pull the injectors using the special tool, that of course was no longer available. I approached the task with much wimpering (as you uncle would say!).
After removing the two nuts that secured each injector and disconnecting the fuel header, I puzzled over how to jury rig a pulling tool to get them out. After much head scratching and fussing I tried an experiment using tools I already had...
Each one come out using two fingers... Sometimes, things are as easy as we wish they always were!
Somewhere around Eleuthera, Bahamas