Wilesco produce thrir roller and traction engines in kit form, painted etc., just screw together. about £100
Mercer produce a complete kit for a traction engine in approx 16mm scale. No machining required, just painting and assembly, but you then have scope to turn your own custom parts, such as a solid flywheel, and modify it from meths fired to gas as you see fit.
It is actually quite a nice little engine, although mine seems to vibrate out of gear when running round the garden. Expensive though, about £350 for the basic kit.
Geof Winwood is the agent
There is a link to an "Old Glory" Review of the kit.
I would suggest one of these as a starting point, before embarking on a scratch built engine such as Minnie, or a 1 1/2" Alchin type, my Minnie castings have been in the box for 18 months :-(