Nice tractor. Looks like someone replaced a broken manifold with a custum fabricated one. I didn't know Bolens had made a diesel in that series of tractor. That's something for me to watch for locally.
Bolens built, depending on how you want to count it, 3 or 4 diesel tractors.
The HT20D and HT22 used the Wisconsin WD2-860 2-cyl air cooled diesel. The engine was actually built by Ducati. Here is where the 3 or 4 comes in as they were identical, except that the HT20D was for the US, and the HT22 was for export. What is interesting is that even with the same engine, the HT20D is listed as having 19.9 hp. and the HT22 as having 22 hp.
The HDT1000, which was supposed to be for export only used the water cooled Mitsubishi K3D-61FG engine, rated at 22 Hp.
Those 3 models were all based off of the HT23. The HT20D and HT22 were based on the 2389, and the HDT1000 was based on the 2389s which had the power steering, cruise control and aux hydro cooler as standard equipment.
The DGT1700 had the water cooled Mitsubishi L3A-61BG engine rated at 17 hp.
All of the other 'Bolens' diesels were built by Iseki, and had a mix of Iseki, Mitsubishi, and Isuzu engines.
This is a late reply, but I just came across the parts manual for the DGT1700--the manifold in the parts manual looks just like the one in the picture--a heavy plate with a tube-like collector... it may be original.
I'll upload the manual for it as soon as I run an OCR on it so you can search the text in it.