Hate it when no one answers a question... for 6 years?
There are three small Echos that share the same chassis: the 280e/280ep, the 290evl and the 300evl. The 280 and 290 are at 27.9cc, the 300 is at 32.9cc. But the 300 piston will not go on the 280 connecting rod because of different piston pins. Need to use whole top end & bottom end of the 300. The handle setup of the 280 bolts right up with a couple of mods to the 300 crankcase.
The 300 throttle cable has to be removed and the hole through the back of the carb chamber where it used to go gets plugged. I used a short allen screw and nut, locktited. Then a hole has to be drilled though the top of the carb chamber for the 280 throttle linkage and its grommet.
I've done one of these for the fun of it and its a nice little 33cc arborist saw now.