Peter,
The cam warnings are for the junky Indian made clones, not the real McCoy British made Lister twins from Dursley. You mentioned air cooled triples. These would undoubtedly be real Listers and those would be very good engines. Those are premium genset polant engines of no doubt. 100 degrees is not a problem IMO as long as you have adequate airflow available and keep the lube oil clean and topped up. Unlike the old CS design engines which require a buddy to hold their hand and drop oil on their ouside rocker arms and pushrods daily the air cooled tripple can be left unattended for long run times at remote telecom sites for example if safety stuff and a large oil pan is part of the setup.
My suggestion for a big single is based on practicality and cost. It will be a lot of machine shop expense and parts to rebuild a 3 cylinder Lister when the time comes. For the same cost you can probably buy a totally new replacement Changfa engine. You could certainly replace the complete head and valves as a unit, cylinder sleeve, piston and rings, replace the bearings, gaskets, crank, cam, injector and injection pump for less than what a Canadian engine rebuilder in my neck of the woods would charge you to rebuild that 3 cylinder Lister. Of course a Changfa rebuild just does not need all that new stuff. If you merely replace the rings, run a glaze breaking stone into the cylinder bore, do the valves yourself, replace the head gasket and put in a new injector nozzle, and all these service parts come with the engine as spares BTW when you buy one new, you can get away easy and without spending money. You will be good after just an afternoon's work for another 10k hours of run time. Disadvantages of the large single will be noise and possibly some power flicker, although there are ways to reduce both of these. The attraction for me is the ease of service and reduced cash outlay. Reliability and service life are high. Fuel efficiency on this large changfa DI engine is also very good and better than the Listeroid or original Lister IDI engines.
Try pat (at) altopro (dot) ca for an engine and a 15 kW ST head if single phase is OK. Tell him I sent you and you will get it almost for free.