I had 634 and 638 engine and now I ride a 632 powered jawa Ts. (TS frame with CZ472 engine (which is Jawa 632, nothing else)
The 634 and 632 are basically the same, the 632 having 12V alternator.
I belong among those who say the earlier engines (meaing 634-632) were better.
the latter has alu cilinders, and better bearings but also the tipically worse material and machining quality of the late socialist industry. :( I saw 1-2mm differences between the steel cylinder sleeve and the alu casting. Carb mounting bolt thread was drilled into the intake channel. Gearbox shifting mechanism was far weaker (the cast iron shifting fork was replaced with a thinner metal sheet fork, and was prone to bending and breaking. :( Fixing stuck gearbox is only possible by splitting the engine. After 2-3 gearbox failures, and home-garage forced splittings, the engine is never good again. There is a proper way to do it, but most owners or mechanics did not bother with that. :( So the 638 is not considered to be a very reliable bike. I put 19000km on my 634, had continous trouble with ignition (setting the point in every 600km) but that was a 10 min job, and mostly due to the bad quality aftermarket points. I had to replace the clutch side crank seal once, that was all. That took me an afternoon. Apart from that, countless hours or oiling, bowden failures, clutch setting.
But never had problem with gearbox, carb or electrics. after 33000 on the clock, the engine needs big end bearings fixed up, so it's in a box.
The now used 632 is awesome. already 12V so I have a VAPE system on it, and it goes goes goes.
Eh... sorry for the lenghty story. :P