I'd be very curious to hear the advantages of any port work on the cylinder head of a diesel engine that has a top speed of 650 or 850 rpm. The ports in the head seem to be of ample area for such a slow speed. Cam timing fine tuning on the other hand, at least to intended specs seems quite important.
I don't have any test experience to verify on Lister heads and I don't know what kind of CFM they move but I have done lots of port work years past. I use to port small block chevy heads for a well known company. I've also done port work on side draft carburetors, very intricate work.
Its not about "RPM". Any engine that has "low flow" at its designed working RPM is going to run poorly due to low volumetric efficiencies. The swept volume on these engines is pretty good for the valve size. That is a massive size piston. Its going to move a ton of air even at very low RPM. When I use to port heads for a living, The slightest bump in an otherwise perfectly smooth port wall can make a noticeable difference in CFM.
Out of all the heads I have ported over the years, these listeroids are the worst I have ever seen. Mine was really bad. Even just some very basic clean up of casting protrusion should help. Its a good idea anyway since you don't want your motor to ingest a loose piece of casting. I was able to pull off a piece of casting in my intake port with my finger. I would not go as far as smoothing the ports, just clean up anything sticking out or protruding into the runners.
If your hand skills are good, blending the seat area to the port (throat area) would be next. There is risk of damaging the seat area so you have to be very careful. There is no valve shrouding work to do since there is no combustion chamber pocket. I would NOT blend this area. I would leave the valve stem alone.
Make sure there is no protrusions where the intake or exhaust manifolds bolt up that can create turbulence in flow direction only. Keep it simple, just take the large bumps out of the road