A lot of the components you can buy for locos can be used on ready to run, kits or scratch built locos. Some techniques can be seen in these articles:

• Split Chassis & Split Axles - Sandy Harper (Video)
• Sprung Chassis Construction using Slaters sprung hornguides - Bob Alderman
• Sprung Chassis Construction using Meteor cast hornguides - Bob Alderman
• Duchess coupling rods - Tommy Day (Video)
• Slaters wheel preparation - Tommy Day (Video)
• Turning cast iron wheels - Dave Brooks of JPL (Video)
• Valve gear - Bob Alderman (Video)
• https://vimeo.com/439531160/5030801e1cLoco chassis enhancement - The Loco Clinic (Video slide show)