I made a conversion of a small Liliput tank loco many years ago that was not so pretty, but it worked...
Ripped all electronics boards out and just fitted a decoder + some wires in native Märklin style...

Unfortunately I have no experience of their larger models, but I have a SVT137 "Köln" diesel EMU that will be converted some time in the future, and that looks not too difficult if you disregard changeover lights...

I presume the big steamers from Liliput are just like other DC steamers from other manufacturers, and the work should not be too difficult since there is already a DC motor in place and you "just" have to choose a decoder that is good enough for your intentions and mount a slider beneath. If there is a natural place to put the slider, it should be easy - if not, some constructive thinking must be made...

There must be some members who have converted Liliput/Rivarossi/Bachmann or other DC loks here...