https://fxairguns.com/wp-content/uploads/2020/08/wildcat-MkI.pdf here is the entire parts diagram for your MK1 from FX. Highlight all the parts referenced as O-Rings. If you see a size, as an example, listed as 10,5 x 2,5, the commas are periods so it’s a 10.5 inside diameter x 2.5 thickness.
The parts list for o rings will also call out the durometer rating(duro 70, duro 90, etc) and the o ring type(NMB, PUR for polyeurothane, etc)
Go to oringsandmore.com and purchase all the o rings for your gun. These will all be metric o rings.
For the dowty seals, the metal washers with the rubber centers, the sizes are also called out, and you can get those from discounthydraulichose.com. Those are metric, also. The factory ones may still be good, just be careful against over tightening the parts onto them. Lube those rubber centers with a dab of silicone grease.
If you decide you want simple, and are willing to pay three times the amount it’s going to cost you in o rings doing it the way I explained(Look at it this way so it doesn’t hurt the pocketbook too much, you’re paying for convenience, right? All the research was done for you, lol) you can go to captainorings.com and they’ll have it in kit form. They do not include any dowty seals, though, just so you know. Only O rings.
of course doing an eBay search on wildcat Mk1 o rings will get you what you need, along with Huma. But there again, you’re going to pay for convenience.
silicone oil or grease on all o rings only! And use it sparingly, and try not to get silicone oil or grease on the threads of threaded parts like the end caps. They will seize up in time.