I intend to go the stock route with my CMP, but I have seen a number of upgraded CMP's over the years, and although not my cup of tea....some looked really smart. Fitting a more modern diesel is something that has definately been done before, and I think I remember one or two for sale in the UK. But, as David mentioned, you have to avoid going from one mod to another. I think David did a clever job on his 8cwt, but I have also seen less succesfull attempts with bad nose jobs on cab 13's as a result.
A bit more speed would be nice...better braking and handling would even be better.....but you don't really plan to drive 100.000km in a CMP??
Also check the forum, as the subject of Nissan (?) diesels and other engine upgrades have been discussed here before.
Alex