I have a similar problem. I have 5 Dextas, one of them is a Dexta, and the rest are supers. One of my project tractors has similar symptoms. However, I can pull it off and it runs fine. Cranking on it will not start it. (new starter & battery and fast spin)
Compression is good, so good, that Must use 6th gear and must be on asphalt or it will just slide. Runs perfect once started.
perfect flow from tank (new valve with preheater fitting), new pump. It should not be the injector pump, since it not rotary, but inline, but I am leaning that way. Injector pump.
Will follow your thread, if you solve your problem it may help me with mine.