to be honest although that bike looks great, those crf50 (whether genuine or chinese copys) are all pretty soft. I have bent, and have seen several of them bend under not so high duty, while having far less length in the forks and swing arm(leverage). If you were serious about doing a convertion with this style of frame i would purchase a ebay china frame (which is almost identical to that) and alloy extended swing arm.....and make changes from that. It would cost litterally 1/8th of the price listed, for frame and swing arm, while still needing changes yes, but it would need significant ability to mount battery and motor anyway so I would assume changing the frame would be just a formality. I myself am going another route....being I will create a whole frame and swing arm from 20mm round steel, in twin spar style, which will weigh (by length of material needed) around 15-16KGs plus mounts for motor, shock swingarm, and battery. Of course there is still wheels, forks, shock, battery, motors, chain etc etc etc.....it all adds up...
oh another problem with them frames is weight, it always stunned me that they weigh so much yet still have obvious weak spots....inherant of frames that are not cradleing/triangulating. Sorry didnt want to turn anyone off this kind of project, but just point out some things i have already put deap thought into. For those interested look at http://www.fxbikes.com/
for more ideas.