Yes there is several brands of new plugs available. New plugs have the same isolator length as most old plugs, and there is normally not a problem with the close distance other than when mounting and dismounting the plugs. You need 18mm thread and 1/2 inch length of threads (reach) in original heads. 

There is shorter plugs for lawnmowers and small engines, but for what I know, they have 14mm thread and too short reach (usually 9,5mm). Champion J17LM short plug has been used by a few in Indian engines, they are a tiny bit hotter than D16, but the short reach is a problem. It may be possible to modify those for longer reach, remove the gasket and turn the sealing surface down in a lathe, but I haven't tried that. 

Reproduced new heads can be had with 14mm plug threads but I don't know if shorter reach is possible to specify. 

Thread inserts that reduce the 18mm to 14mm is a way, but because of poorer heat transfer through the material and dual threads, it might be necessary to use a one step colder plug than normal. I know also some model engine builders that has been shortening the isolator on spark plugs, but that was for model engines that was run on display stands.

You have a cross reference list here for a Champion D16, normally used on a flathead Indian, but be aware that some of the recommended plugs can be of resistor type or have some other specifications that is not good to use with magneto ignition:

http://www.sparkplug-crossreference.com/convert/CHAMP_PN/D16