Can I use modern control systems such as feedback controllers or DCC?
Feedback controllers are widely used. Modern motors appear to give no problems, but older motors such as those in Triang locomotives may suffer from overheating.
Some modellers are trying out DCC. Provided your locomotives have suitable motors and perform well, and in particular pick up current reliably, there appears to be no problem, but it may be good idea to get your locomotive to work well under DC before converting it to DCC. N scale decoders can usually be fitted, and in some cases OO/HO ones.