http://www.ofna.com/manuals.html
towards the bottom "motor manual" "Force .21 P4, Black T8"
Basically pull every thing of the forward part of the crank (clutch collet and brass bush and the carb ) undo the 4 head bolts remove the head and glow button,
put the piston at the bottom of the liner and stick a finger down there pull up the liner,put some pressure on the exhaust port with your finger and give it a wiggle,Should just slide out 80% of the time. If its stuck you will have to heat up the crank case and use some sort of soft plastic lever (I use an old tooth brush handle with the bristles cut off) to lever it up either from the bottom of the crank case or one of the ports,be very careful as the slightest bur on any of the ports will stuff the piston when its put back together.Once you have the liner out you be able to slide the piston/conrod back and of the crank pin.Then put a nut on the cranks thread(where the clutch normal go's) and using a large screw driver handle give the nut a sharp tap to get the crank out of the bearings.
If your going to replace the bearings your going to have to heat the crank case and using a suitable drift knock them out.To replace the bearing put them in the freezer and heat the case they should then drop right in with little or no force
Hope this helps
Wozza