Vintage replica engine build

Hi, talking to a vintage car group who want to build a late 19th century, single cylinder engine based on very little other than a few photos and descriptions. They have a wealth of knowledge about vintage engines - most are retired engineers - and I'm discussing maybe doing the CAD and drawings for them.

From my experience, when you get to the moving metal stage the budget starts evaporating very quickly.

If we have the CAD models what would you suggest as cost effective ways to 'test' the engine and improve the likelyhood of the real thing running with least numbers of physical interactions and mods?

Cheers, Sean