As Dan said, you will have to be sure your new car fits the track. On the Harken site, under Technical, or Catalogs they have some diagrams with dimensions of different tracks. You can then cross reference any used car you buy, & see if it fits your track.
http://viewer.zmags.com/publication/b45b96ae#/b45b96ae/102
The Harken site will tell you what track # is compatible with what car. There are some warnings about the newer captive bearing cars not fitting the older tracks.
If you replace the track, check what the spacing for the holes is. It would be a pain to have to redrill all those holes.
Other considerations are traveler cleat base is supposed to be level, check the Nacra Manuals, I think a boomless system has a slightly different track orientation than the boomed models. A swiveling top on the car is not required, as the connection between car & block will allow for the change in angles that occurs as you sheet hard. you can see that in the photo of my 5.7 setup. My boat has the longer Harken car,(appx 4") they make a shorter one too, & it is easy to get replacement bearings for them if needed. You want your car to slide easily, so that it will move unassisted, even in light winds. Buy or borrow a car loader if you get a system with non captive bearings. It makes life easier. I've done it without one, didn't know the shaving cream trick at the time.
I believe your 5.2 uses a slightly different traveler sheet arrangement.
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Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
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