I really think this would be a great upgrade for the Listers in place of the cam idler if possible, or some place lower down in the block if room permitted.
More observations/thoughts:
The easiest place to mount an external pump is at the other end of the camshaft where there are no other bits in the way.
The Lister engines have a steel/tin cover which could be very amenable to having a 14g (2mm) stiffener inside with a couple of hank bushes, bolt the pump on here, a small tongue/slot coupling into the end of the cam and off you go!
While you were there, you could arrange a decent oil feed to that end camshaft bearing which always suffers through lack of lubrication.
Peter