Having a seperate head will allow you to alter your compression ratio and optimize the squish band and combustion chamber shape without the risk of moving the intake or exhaust port timing beyond where you want it.
Where`s y'trA when we need him to `splain this? I have no idea what I`m talking about.
I would imagine a 2piece head would work better. If you can us the 2 piece design to increase or change compression and get higher performance from a saw, then do it, if not then dont, its your choice, not mine.
the reason for a head is quite simple , like Jokers said you can change your compression without moving your port degrees plus some stock jug donot allow you to make alot of compression because there dome is to big, also you have to watch that when you cut the bottom of a stock jug down to gain compression that you donot cut so much off that the ears on the jug donot break off because they are to thin now to handle the compression.
I find it easier to port the jug with the head off to