Graham,
A general description of the engines (side-valve or over-head valve, threaded ports or flanged ports, fore and aft or transvers or vertical, single or twin, etc) and particularly the engine prefix code (and perhaps the serial number) would go a long ways to help folks identify which of the many types of engines (motorcycle, moped, industrial, car, and from 1907 to 1956) produced by Douglas your engines might be.
-Doug