Yes, it has been done. There are even a few build threads on these forums of people building these conversions. Some of the small ones can be flown on the Horizon hobby UMX brick and motors. The larger ones can take more standard electronics, sized accordingly of course.
Guillows kits have a reputation for using balsa that's on the heavy side, so build with weight in mind.
Edit: Unless you're referring to the 4 piece sheet-balsa planes you can buy for a few bucks. I don't know of anyone motorizing those. I have seen people build larger foam versions of them.