A longtime sidecar partnered with scooters is the Cosy (lately supplemented or replaced by the Inder):
however, at the webpage: http://www.scooter-sidecars.com/
there is a sizable listing of manufacturers.
As to "taking off and replacing" a sidecar; THAT may not be all too easy or practical. Read from the following site:
sidecar can not be detached from the scooter easily. To remove it,
you must remove the center bolt engine mounting, remove the sidecar
frame work and center stand, and replace the original vespa center
bolt engine mounting."
Keep in mind that any powered-twowheeler with a sidecar is going to be very different from just the solo twowheeler. How does adding a passenger and fully-loaded topbox to your present scoot change its performance?
An outfit tends to handle similar to a under-powered gokart, with the left-front wheel missing, and the driver sitting up on top of his seat backrest. Don't misunderstand me . . . they can be practical and a lot of fun-- but an outfit is a different kettle of fish to a solo.