Like Alex I don't know the difference between the JXLD and JXD engines. I did find a reference to an experimental replacement of the JXD in an armourd car with a JXLD, so they must have been at least similar in mounting points and general size. Has anyone seen a table that decodes the letter code for the Hercules engines?
It seems to me that if you can find the manual for the exact engine it is a better choice than one that is only similar. I don't know if the military published a manual for the JXLD. Does your generator have a data plate that lists the manuals that apply to the generator set? There seem to be several online sellers of old tractor manuals who list manuals for the JXLD. I have not bought from any of them to be able to comment on their service. One of them is
http://www.antiquetractors.com/conte...nuals_list.htm