@foosa,do u want to install that engine in your chicom bike or do you have a frame of XL250. for your current bike, i think you should get the engine of Suzuki GN 250, it is a single cylinder engine and very fuel efficient and parts are very easy available even new engine part were available as they were issued to Punjab Police.
if your current chasis can support, which i doubt, you can go for ZZR/EX/GPX 250 engine, which are twin cylinder DOHC and 8 valves engine.they will be available between 15 to 20 k. it is a small engine with a very narrow top end and can be installed but it is water cooled and u will have to fabricate some mounts for radiator plus some modification to
grp panels of bike to ensure free flow of air.
and if you have a taste for two strokes, then a twin engine from Suzuki RG 250 or Yamaha Tzr or Honda NSR 250 will super fine and no engine in this capacity can touch the powerband of a stroker twin.