Andy,
Been about 12 yrs since I sold my L's but on them I used a plain ol' clip on light that they sell in just about any hdwr, home center, etc. They have various clamps but you are after the proper reflector. At the time I used them the reflector was about 10" in diameter but lately Ham Dupa had some that had 6" ones made out of some soft bendable (adaptable) metal.
I just clamped the light so the reflector was as close to the block as possible. If I recall it clamped on the side rail of the rear hitch frame and shone it onto the side of the lower aluminum crankcase. I generally had 75watt "rough service" bulbs in those lights and if it was real windy I had a tarp to throw over it (which I really don't think it needed). Good thing about an L model there's nothing flammable or meltable on the side of the engine. Putting the light under and shining up probably will work an ounce or two better what with heat rising lately!
Disclaimer: These two Grande Dames of mine had the original "T Head" Gravely engines and not the later Kohler or whatever came next... In any case 75watts is plenty in this area. I wouldn't be afraid to go 150watts on those engines for a "quickie" or in Northern climes.
Now if I was going to do this with my Deere Kawi twin I would see which side of the engine has the clearest area for the light as I can't get under it. I think a side mount will heat it enough but I don't need a heater, it starts better than I do. Keep the bulb non-melting distance away from any rubber or plastic engine stuff. Some of the halogen spotlight bulbs direct some real good heat in a narrow band with their own reflector and might be a good try.
I started using light bulbs on a '51 Ford pickup but only to heat the battery and increase cranking power. Worked on my VW Microbus also.
Let us know what happens...
