One way forward is to modify the wheels to suit current RTR track. I have done this 'by eye' on my older items using nothing more sophisticated than a power drill, a collett made from a nut and bolt, and an old tool stone, to cut down the height and depth of the flanges. Retaining the original wheels maintains the character of the older locos in a way that bushing the chassis and fitting modern replacement wheels does not: but that is another technique that is available.
Reasons for adapting the older locos and stock to current track, as opposed to using older standard track: running is much more reliable, appearance is much superior, when stock is taken elsewhere it will usually encounter fairly modern track. Unless you are able to set up completely separate tracks, going to an earlier coarser track standard to accomodate the older items will cause problems with running some of your newer items: the finer wheels left completely uncontrolled in the larger flangeways derailing regularly on facing points for example.
Reasons for adapting the older locos and stock to current track, as opposed to using older standard track: running is much more reliable, appearance is much superior, when stock is taken elsewhere it will usually encounter fairly modern track. Unless you are able to set up completely separate tracks, going to an earlier coarser track standard to accomodate the older items will cause problems with running some of your newer items: the finer wheels left completely uncontrolled in the larger flangeways derailing regularly on facing points for example.