It's unlikely to be the injection pump if it runs fine once started.
Things that affect diesel starting are compression, injection timing, spray pattern, valve timing ...also for an engine that runs well once started, air getting into the injection pump/fuel line is a common problem. The latter possibility becomes more likely if it is easy to start if run recently...as it would take some time for the air to slowly get into the line. Slow weeps of diesel fuel in lines, filter or at IP are also an indicator of air being your culpret.
The spray pattern should be checked as it's easy. Pull the injector, loosen the high pressure line and swing it away from the cylinder, reinstall injector. Being careful to avoid injecting fuel into your skin, use a coffee can or similar and watch the pattern while cranking. It should be a pulse of fine fog-like particles. A big of crud on the tip of the injector can foul things up, and yes, you could have a problem with a 2 mos old injector nozzle.
For valve and IP timing, you should check the manual. It's possible that having it off a bit will still allow it to run OK, but be hard to start. While your injector is out and connected, you can slowly crank and see where it's firing relative to TDC on the flywheel, assuming it's marked. Retarded injection timing will normally cause a loss of power and sluggishness to respond to throttle. Advanced injection timing will cause more diesel knock.
I don't have an ST-1 so these are only generalities, but I've seen them all on my other diesel engines.
The exhaust valve is now suspect since your failure was started by the decompressor getting out of adjustment. I'd sure listen to that carefully while it was pulled through compression, even going as far as pulling the exhaust pipe or manifold to hear better.