As you know I have a 13BT I rebuilt last summer. A 13BT is a 12HT minus 2 cylinders basically. The 13BT was made for fewer years than the 12HT.
I will say there are still a surprising amount of parts I was able to get. Engines Australia has a full rebuild kit for the 13BT and 12HT, plus sleeves, Alfin pistons and basically everything you need.
there are also achunk of parts you won’t Be able to find new. Head, block Probabaly the camshaft and all sorts of bits of pipes, hoses little things you may never need to replace. You would have to make/modify search some of those parts.
that was a long winded way to say if you want a 12HT, you can find one and spare or used parts. Australia is a great place to look. It will be harder to find than 1HZ or other parts,but the majority you should be able to get. It also matches up to your H55f already.
another option if you go 12HT is to stock pile parts like the 70 series folks do. Get a spare head, all sorts of parts just to have them and not have to worry about finding later. A lot of 2H parts cross over, I’ve heard valves, can, lifters, valve train etc.
Performance wise, you will be quite impressed with a 12HT in a 40. People always look at specs on paper and say it only has 134hp 230tq, but that’s deceiving. True it’s no modern engine or built Cummins but it will scoot a 40 along quite well.
My 13bt carries my BJ74 which is heavier than a 40 nicely stock. I have power available in upgraded turbo, inter cooler, fueling.
gturbo, Munro racingturbos and wink 4x4 build custom turbos for a 12HT. All sorts of options. The exchange rate is pretty good Now.
wink 4x4 turbo and12ht in a troopy. Looks to move pretty good and sounds awesome