Hiya Motomedic........ its indeed possible.. I dont know if i dare post on this topic! the last time I got involved in a "discussion" about sidecars it deteriorated into a war with one member getting a 3 month ban, another getting 9 points ( 10 is a ban) and stern warnings all round.
mind you the 3 months Ban was over in 4 Days..... so that is one good thing eh Caminando?

Right on with the Post!

stuff buying one......MAKE ONE!
Google trials sidecars and manufacturers, copy the designs, the wheel any 125cc trail bike front wheel with a spindle will do, make up some spacers. bish bash bosh job done.
the frame, lightish 25mm tubing with a 3mm wall should do it, (I used box tubing on a trike Ibuilt with a metro engine and a CB125 front end, that was plenty strong, it even passed SVA test)

Suspension on the sidecar may be a prob, again, look what the trials boys r doin. i dont think there is any on a trials setup.

mounting to the bike, 3 mounts is all you need. I would try to come from a front engine mount, a passenger footpeg mount and probably another, but im writing this without a good clear idea of what your looking at doing so its a bit sketchy and from memory.

Dont forget toe in and lean in to make it handle properly!

Martyn