RE: ZR.30 in XL video
The backing plate should work. They have the same stroke.
I found this kind of interesting, according to this page, both 30s use the same crankshaft.
[link=http://aedownloads.com/press/in_022309_lrpengines.pdf]Linky[/link]
The 28 has a different crank listed, wonder what the difference is.... Glen