I haven't seen much for sale in the UK recently, but found this nice condition engine for sale on ebay in France, it's an early Model F with 6.921 to 1 ratio gearbox for 900 R.P.M. output shaft speed.
Decals on this engine, not many had the Model printed on the O&R eagle logo decal, I've noticed that it isn't stamped into the crankcase on this engine either.
The gearbox for the Model F has no clutch fitted, a Model J was also made if you needed the same specification but with a clutch, Model J engines seem to be more common than the Model F. Below is part of the 1961 price list showing these two models.
David