G'day Mate,
The main differences between the "LA's" & the "FX's" is the LA has plastic backplates, which can come loose easily, so check that it is not loose, & the FX has a metal backplate which won't come loose.
Also the FX has a 2 needle carby, where as the LA has an air bleed carby, the FX usually stays in tune better than the LA, because the air bleed hole is on the front of the carby & can get dirt stuck in the hole, causing problems. And of couse the LA is a plain bearing engine, while the FX has ball bearings. They are both good reliable engines, but all the LA's have the problem with the backplate.