Kevin, I converted 2 of the Stainz locos, vaguely Decauville, one with open cab, one with larger Progress type cab. I also converted a Piko 060 again with later Progress type cab. All it involved was new cab, new side tanks and new chimney. Not perfect , one reason I cosidered 3D printing untill I saw the price. May still do so if I build another layout.
For smaller locos I used the USA Trains motor bogie, not so easy to get hold of now, but they are on this website, along with the Hartland ones.http://www.g-bits.co.uk/prod.html
I will have to dig out some pictures.
As I said I had a loading gauge limitation, width was restricted, so locos are no wider than original LGB ones. Would love to have a protected Simplex, but even in 7/8 they are wide, one reason I might build another layout. I do have a square framed 20hp Simplex, built on a USA Trains chassis. Far too many locos for a small layout, so if I do build another it will have to be a loco depot.
I have both versions of 1st radius LGB points, the original had break in point blades in middle not next to the frog. For inset track9pre 3D printed of course), I prefered the older points but had to settle for newer ones. Both can have electrical problems as the depend on weight to make sure contact between blades and rails are maintained, and it is not easy to solder to heavy weight brass. Anything I solder on is to rail joiners, when they are not attached torail, as it acts as a big heat sink!
Although not on this layout, I have tried out Peco G gauge flexi track. No need to use special rail joiners though, as a cut just above flat base of LGB rail is right height to fit a standard Peco code 250 rail joiner. I think code 200 rail track(G1) could also be joined by a similar method. Cut would be higher, or could join LGB rail to a short length of code 250, then connect this to code 200 by similar method. This is what I would do if I was using my 3D printed track as that uses code 200 rail. LGB wheels happily go though code 200 rail track.I have even tested inonone of my 3D printed points, where back to back on wheels is potentilly bigger problem, and resulted in my having to adopt code 200 rail instead of code 100 as used on all my other track. Will make mixed gauge more difficult, also my SM32 is also code 200.
I think the 7/8 group recommend the smaller profile rail. The aim originally was to have a pseudo finescale standard, so people could run models on layouts from other members.I am not sure how widespread those standards are applied now.