Answers

I cannot say that I've ever done this or known anyone who has, however, I found this helpful step by step on the internet from someone who says it's relatively easy and can be very cost effective:

http://www.instructables.com/id/Converting-a-generator-to-run-on-propane/

For most gensets, simply changing the orifice and a few adjustments will allow the conversion.

Here are links with instructios and video:

http://www.thegoodsurvivalist.com/how-to-convert-your-generator-to-natural-gas-or-propane-for-5/

http://www.instructables.com/id/Converting-a-generator-to-run-on-propane/