Hi. A Swinging arm engine has an independent gearbox, consequently it is much longer than the plunger engine and gearbox. Fitting a swinging arm engine into a plunger frame is a non-starter without somehow stretching the frame. The plunger engine, in my opinion, is just as good. In fact my A10 plunger with original mag and carb absolutely flies, has loads of grunt and is easily my most pleasurable bike to ride (I have five A7/A10's). The last A10 plunger engine was built in 1957. I would not recommend fitting one of the early 'longstroke' A7 engines though they would fit, as parts are getting scarce and the later engines had some significant design improvements. I would recommend looking for an engine with prefix ZA10 or BA10 or BA7 but make sure the crankcases have the four large threaded bosses and rectangular rib to accommodate the gearbox studs and casting.