H-500, 269HM Allison C-18 to C-20 upgrade experience?

Looking for opinions from folks who have done this. We have three OH-6A's with
C-18's and are nearing our last "in the can" engine. What is the availability
of C-20's to public use operators such as police? Who has done the mod and can
tell me what is required (new short shaft, ect?). I am fully aware that it is
a better engine, but what about the DOD flow of PLL?
Most interested in the availability of surplus C-20's. Please email me direct.
Hugh Mills
KCMOPD