You can get the boards directly from Lionel. The website is lionelsupport.com. Replacement parts.
There are 3 generations of Legacy and apparently the newest releases have a 4th generation but not much information about those yet.
1st generation around 2008 used a R4LC and DCDS with Railsounds 5.5. Boards look identical to Odyssey but with different code.
2nd generation is RCMC with Railsounds 6 or Railsounds Lite.
Then RCDR and BEMC with Railsounds Lite which I believe is what is used in Flyer as well as some O scale. I have not worked on any Flyer so someone else will have to confirm.
Search the forum on Legacy Upgrades for threads on doing this. Examples of all three generations are described.
Keep in mind there are no kits and minimal information on wiring the boards. You will have make all of your own wiring harnesses. Had you inquired a few weeks ago you could have gotten the parts at half price. Now you have to wait another year to get that deal.