In the usual engine castings are usually needed for the wheels, cylinders, cylinder covers and valve chests and the eccentric straps. Some other components such as chimneys, domes, axleboxes and hornways are usefully done with castings. Some of these may be of aluminium, while fabrication from steel or brass is often a good option and can reduce the cost. If you want to try your hand at pattern making, you can have castings done to your own patterns. The best place to get help is by joining a club.
Most model engineers are a helpful lot who can talk a lot too.
You need to be a member to be officially issued a boiler certificate but don't let that prevent you from getting the inspectors involved. They can often give good advice that saves you a lot of work. And then when you join a club, the records will be available for a boiler ticket to be able to be issued. Most clubs expect new members to participate in the activities of the club from running miniature trains to mowing the grass, and the reward for this is lots of conversation and useful help not to mention access to the club's track, libraries and the like. You can find lists of societies on the web.