You must be young, as spark plugs have been standard threads since the 1980's at least on American vehicles. Here is a picture of what I'm talking about. They are straight thread NPT . Any hydraulic shop will have them.
Ha! I like hearing that. I don't feel that young, and I was driving by the end of the 80s... And not far from you in fact. I grew up on the east side, in Issaquah.
I'll admit to no experience on American cars. However, I do have an american engine in one my vehicles though (chevy 350), and recalled being confused at the presence of metric on the engine. I just checked the Machinist's Handbook and found that the SAE standard for spark plugs lists only metric threads. The spark plugs for that 350 (which I just checked) have 14mm threads. A little more research helped me remember what I was confused about.
The SBC has had 14mm spark plug threads since it's beginning in 1955. But the seat has changed. All of the wrench sizes are NOT metric. They are either 13/16 (early) or 5/8 (late).
So what had befuddled me years ago was that spark plugs used metric threads and standard wrenches. I believe the same is true on foreign cars (but I haven't checked and could be wrong).
I also checked all the straight pipe threads (NPS) in the Machinist's Handbook (and all the other threads I could find).
Straight pipe sizes jump from 3/4 to 1"
British Standard Pipe threads aren't tapered, but still won't work, they have Whitworth threadform, and the closest size is 7/8"
In metric threads, the machinist's handbook doesn't even list M20x1.25. "normal" thread pitches for that diameter are 1, 1.5, and 2.5mm.
So it appears this is a pretty uncommon thread.
Anyway, last night I decided I had many other placed to spend my time, so I cut the end of the injection nozzle, removed the contents and tapped the ID for 1/4 NPT (hole was a little too large for that) and glued in a pipe plug. That will seal the hole for now with the proper threads. And I can focus my attention on things like brakes and fuel for now.
If my OCD fires up again on this, I might just machine some plugs. But it's pretty far back on the list right now...