Would would think seals on back of the regulator would be worth changing
Aside from the few sensors mentioned, you really need to get the codes/live data - you can throw parts at it if you like, but really you need to diagnose the issue.
Get it on a reader and check the fuel pressure on the low side and the high side, then you can rule in/out fuel getting up to the rail at the right pressures. It might be the O ring at the HPFP regulator, but it might not!
Then progress to checking the injectors, and so on.
The likely sensors would be the camshaft sensor, which will prevent ignition, and the fuel pressure sensor on the end of the rail, which can cause running issues. They can be tested as I said, or replaced, but be aware of cheap parts - I replaced 3 camshaft sensors on my F1… the original, a cheap new one which gave me issues, then a ‘proper’ new one.
Unplug the MAF sensor and see if it runs better or worse.