Another vote for purchasing a new, retailer sold (rather than used, warrantee-less) cab-1L/base. Not all that expensive compared with other solutions. Strikingly cheaper (by hundreds of dollars) than installing PS3 and purchasing DCS, for one thing. Charles Ro's price is about $215.
Furthermore, I know of no one who has modified an existing locomotive to install LionChief or LionChief Plus electronics, probably because Lionel has not encouraged sales of separate locomotive innards and the corresponding remotes.
JohnGaltLines has worked on a method for using both TMCC/Legacy and LionChief simultaneously from one remote, but to my knowledge he is not providing that as a commercial product as yet.