Strangely enough, I went the other route and worked to the original drawings in Imperial, but converted the threads to metric. As it happens, the metric sizes are pretty close to the Imperial sizes, although I used drill & tap tables to get a closer fit when drilling. So, I found that 2-56 becomes M3 x 0.5; 10-24 becomes M5 x 0.7 and 1/4-20 becomes M6 x 1.0.
Other changes I made were to make phosphor bronze bearing sleeves for the flywheel / crank assembly. They're secured by M4 set screws, which can be removed to reveal oiling ports for the bearings.
The great thing about this engine is that it's big. Don't need o be a watchmaker to build it, but it's still within the capabilities of very modestly sized lathes and mills - although I have to admit to having a 5" 4-jaw chuck for my little 7x10 Harbor Freight lathe.
Eric