Yes, the threads look straight. The hole is about 1 inch across. Would you know what type of thread?
Yes the 87MM Standard Motor Company engine
The following image is a portion of the parts diagram for the 87MM Standard Motor Company engine, which shows the location of the plug but makes no mention of either the plug or it's part number. My guess would be that it's just a regular pipe plug. If you can determine the thread count and whether or not the threads are tapered, you could probably buy one at a hardware or automotive parts store.
I take a closer look at the threads tonight. I think the regular pipe plug will do the trick.
I don't know for certain about the 87MM Standard Motor Company engine, but most engines and/or cylinder heads I've worked on used pipe plugs with tapered threads. In the United States the tapered threads are known as National Pipe Tapered or NPT for short, but I'm certain it's different in Europe where British Standard Pipe or BSP threads may be used.