the 150cc engine in your ATV produces 10BHP whereas a 250cc inline four cylinder liquid cooled heavy bike engine dishes out 40BHP. How will harness that much power, how will the chassis coup up with 4 times the power is was originally designed to handle. You will need bigger better brakes, better tyre..... ATV ki handling ka tow aapko pata hee hai...sooch loo bhai...sirf engine change nahi hoga.......itne kharchay may shayad aapko purani 250cc heavy hee mil jaye. If money is not an issue is can be a very good project.
Lahore ka tow pata nahi.....about a month ago I found one engine at Ranchorline Karachi, its was from Honda CBR250RR (inline 4cylinder 6 speed 40BHP and rev to 19,000RPM). Price tag was Rs.40k.