Make up plugs on the lathe to accurately fit the mounting holes in the engine’s casings. The plugs should have a shoulder or flange to help locate them in the holes. The faces of the plugs should be flat and have a shallow pointed hole drilled in them, just enough to locate the legs of your dividers.
Using the dividers, measure in pairs, the centre to centre distances of all the holes/plugs and either make a CAD drawing or paper layout from their intersecting arcs. If you’ve made decent plugs (keeping their ends in a common plane) and measure carefully, the results will be good enough to construct the chassis mounts from (they are probably likely to be welded on and will pull slightly as they cool anyway).
Use numerous pairs of arcs to confirm the mounting hole positions from as many different pairs of plugs as possible and stay in plane to eliminate third axis trigonometric errors.
Measuring Engine Mounts From Photos
Taking measurements from a photo can be problematic and really needs proper software to digitize the image which will have at least some parallax errors and will need to be scaled before any of the data can be used.
At the very least, mount the camera on a tripod or box and place the camera (preferably digital) as far as possible from the engine. A close up will have too much spherical aberration to be accurate. Since you don’t need good resolution, it won’t matter if you have to blow the picture up, just as long as the area you want is in the middle of the image.
Make sure the camera is square on to the engine and the lens height should be as close to the mid height of the engine as possible.