One of the great things about TMNR is that it is all 'low tech' engineering and so both easy and fun to restore or upgrade.
The bodies are all moulded plastic shells, split into a front and a rear section and attached to a steel chassis. This runs on bogies which are attached to the chassis by pivots retained by split pins.
The range of components used by all the stock items - coaches and locos - is quite limited and whilst Imperial standards modern metric bolts work equally well.
A few components are not easy to replace - the rubber joiner section which fit between the coach & loco 'halfs' for example - but most can be obtained from any hardware supplier.
My advice would be to use the best quality parts one can afford.
This is what we started with...