Welcome to the DOW NeilW.
There has been three Ben Peterson book builder systems that I can recall done out to engines running here on the DOW.
Two Canadian. One east coast USA. All in bed asleep now. So’s you’ll have to wait at least 12 hours to a day or two for them to read this and respond.
In the meantime view these from one of them:
So as you can see you can fab, make up a system using general dimensions. Nothing is precise critical except gas tight welds where book called for.
Then read here . . . it’s long with lots of detailed construction pictures and some operational videos:
I started you off at his first construction pictures. Shows one of the two holes you are asking about. Once in-topic you can use the thin blue line righthand scroll bar and skip down to post #53 for more detailed pictures of the side holes. And their purposes. Again, nothing is precisely critical as long as the functions do perform.
Regards
Steve Unruh