There is no adding a valve train to a 2 stroke, but the 4 stroke valve train adds complexity and weight. Emissions has killed off 2 strokes as they are inherently "dirty," but I love em. Yamaha 350, Honda 500cc Grand Prix champion,on and on....oh, sorry this is a lawnmower forum.
My first trimmer was a big box Ryobi 790R which cost me (I thought) big bucks , $200,but seemed closest to commercial with my limited knowledge at the time (2003). It came with a 31 cc 2 stroker, the clickit shaft and removable brush blade. I cannot count the acres (yep , acres of weeds, grass and dirt I moved with that great trimmer. Never a problem for 10 years and then mag pick up vs flywheel caused me to try to trouble shoot it. MTD (Ryobi) did not want the unit repaired so the tank, coil and gas lines were prohibitedly expensive, and it was difficult to disassemble. I waited and searched, and online was able to see that the 31cc demon 2 stroke still lived in a sears straight shaft , bump head with the clickit ,and the old style gas tank. (Still available online for $118). I have used it for several hours now and the motor is still great. Different head (two level, less string). the old Ryobi clickit shaft allows use of the the old well worn bump heads and the brush blade. It a low end I love so far.
Regarding weight the shaft material can add weight, steel (mine) vs an aluminum Lowes Husky I once looked at for $250.
