For almost 20 years I woned a Stag and for a variety of reasons it was rarely used and kept in store. But I would start the engine up every 3 months or so and wrm it up whilst staionary. Doing so eventually bu66ered the bearings at 33000 miles because it was always being run from cold and after oil had drained away.

In regular use I doubt it makes much difference how you warm the engine up. The engines in the Morgan are not high performance or highly stressed unlike say an R1 engine. So I doubt they need any more consideration than the average shopping car. My only rule would be - if you put it away for the winter first change the oil and then leave the car alone until you are going to use it regularly once more.