Historically, these mills started with 1600cc engines. 69mm crank and 85.5mm cylinders.
Later they changed to 90.5mm cylinders which didn’t require any major modifications to the case, but gave you a bit more power. These were called “1700cc” engines even though they are technically 1641cc.
In the mid 80s (maybe) the engines went to 1835cc which was the same 69mm crank, but the case was further modified to take the larger 92mm cylinders. These work, but with less metal, they are more prone to overheating and all the issues that come with that.

For our “restart” as we get production of parts and mills going again, we are going with a 1759cc engine as our standard. This is a 74mm long stroke crank (more torque), 1.25 ratio rocker arms, and 87mm cylinders (more material, better cooling).