Way back in 1994 I gave into the dark side by going from road to MTB. I bought a Trek 930. I love that bike and I still have it. I have had a dozen proper MTBs since then. Last year, I went to the dark side again - by buying my first EMTB.
My advice is to restore your Lefty to vintage condition and gaze at it from time to time. Next, go buy a new EMTB. Just do it.
I know some are brand collectors, 2014 is still kinda new.
Cannondale will not warranty any frame that has had a motor strapped to it. I think a motor on a frame cdale is asking for trouble. There's a reason they have been called "Crack N Fail" for a number of years.