Pretty much all of the Breck/Keystone area is USFS, so not many legal trails. There are some motorized ATV trails (Jeep road width but really rocky/technical) off Tiger road but I wouldn't probably prioritize that just because you can ride an e-bike. Essentially all the singletrack is off limits.
As I recall a few years ago they were still debating whether to allow e-bikes on the Dillon reservoir bike path!
-Walt