United Trail is a very Good platform for trail lovers but it is coupled with Chinese low quality and poor assembly.
It has engine bay big enough to accommodate a 250~300cc engines easily but in air cooled variant, for water Cooled or oil cooled breed one has to get a fitting fabricated in front for the Radiator/condenser.
1 - A standard rickshaw 250cc is available for 36k (local manufactured) in brand new condition from Lahore market as per last info but bikes same engine in bike configuration is 60k brand new (Chinese imported).
2 - In used same engine can be found as low as 18~20k for rickshaw variant, same engine with bike configuration can be found for 30k or some time even more..
3 - Air cooled variants are even more expensive and rare to find.
United trail has a tendency to fry its wiring or batter for no reason and around 4~5 incidents have been noticed in close circle recently, including this engine swapped untied as its wiring fried and it fried like 4~5 CDIs as well due to short circuit... its fuse and wire harness is of very low quality and fries up quickly.
So my advise would be that if you can monitor your bike when mechanic works on it and if you know basics of bikes then only go for swap its not a easy hobby ...