The only real difference between an early and late XXL is the clutchbell. I think the early one is 19/26 and the later one (and the same as used on the LST2) is 18/25 I believe. The transmission is the same in all XXL variants I believe.
The chronology of the LST trucks is: original LST, Raminator/Aftershock (same as LST except FOC transmission), LST2, XXL, XXL2 (GAS model), and XXL2-E. Difference between LST/Aftershock/Raminator and LST2 and XXLs - beefier axles, suspension, and spur gear hubs. Differences between LST/LST2 and XXL trucks is XXL has longer chassis and transmission is FOC. LST and LST2 have a forward/reverse and hi/low range transmission. A Forward only conversion is common on the LST/LST2 trucks. Shaves a bunch of weight off.
For engines - I'm a bit of a snob. I used to use SH engines (same company makes engines for the Losi trucks), but I now use Italian engines. One of my trucks has a Novarossi legend 28-8RT and the other has a Picco P3 .28. The Novarossi is still for sale and can be had for around $180. Find it at Novarossidirect.com. For exhaust, I would use a Novarossi 41017 manifold and Novarossi 9886SS pipe. This manifold and pipe will suit a stock Legend 28 well. If you want stupid power, get a modified Legend 28-8 from The Nitro Shop. Don't do the pullstart model; get the rotostart model. You will need a rotostart wand and may need to do some custom work to the starter wand to make the Nova starter driver to fit. I can post pictures of this later.