ummmmmmm..yes, you can make the 2wd into a 4wd motor !!. thing is, yu'll need to split the motor, and change out the rear output drive shaft main shaft. and the left side cover. he'll run into a problem with the front frame for the differential though, as there is no welded nuts for the differential to bolt to.You can swap the motors, but you can't make the 2wd motor 4wd. You can swap the 2wd motor into a 4x4 frame, but you won't have a front driveshaft, and thus won't have 4x4.
What's wrong with your 2wd bike, and how cheap is the 4x4 bike?