Well, kind of.
Since the two cylinder bolts are on opposite sides to the echo's cylinder, I had to drill and tap two new cylinder mounting holes. Before this I also had to strengthen the case halves using jb-weld, to give the screws some more material to bite into. The hardest part was getting the cylinder correctly seated onto the case.
The G26 piston and pin went onto the 21's connecting rod without any modification. The only thing I got a bit worried about was the fact that the piston skirt comes very close to touching the counterweights at BDC. But this never gave me any problems. The transfers also need to be matched to the case...but I never got around to doing this. Also, the stroke of the 21 is about 1mm shorter than the zenoah's, but once again this never really caused any problems other than a slight loss of compression.
I think a G23 piston and cylinder would have probably been better, but I already had the spare 26 head kit laying around.