You can get BMX cranks that are short.  It might take a little bit of looking.

Greenspeed trikes in Australia quoted me $120 US to shorten Sugino triple cranks to my specification (I am interested in 125mm to 140mm).  That is how I will do it for my trike.

On the CT you don't need a triple so the cost is a little high maybe.

It is possible to do it yourself, but it is tricky and a milling machine or jig borer is better than a drill press.   You need to fixture it very well to get the pedal axis parallel to the crank axis.  People are doing this using the rethreading taps that Park Tools sells,.  These taps aren't the best for a new set of threads, but you can do it.  Finding a taper tap for this thread is not easy so that it the best option - getting the one from Park and taking you time with the cutting.  It would be best to use the mill to hold the tap rather than trying to do it by hand.

You need to scrounge up a set of cranks with enough meat in the area where you want the new holes.  This is not always easy.

You might want to consider some bolt on shorteners as sold to the tandem crowd so kids can ride the stoker position.  They aren't terribly expensive and you can experiment with different effective lengths.  They increase the Q factor or distance between pedals.  You may or may not like that.  If you are OK with the big Q you might just use the shorteners - who knows?

You can search the trikes archives at ihpva.org for the most comprehensive discussion of this topic.

You also might want to enter your query on the other BikeE and recumbent lists.

Mike