Welcome Adam.
Looks like you've got some work ahead of you. You stand to learn a lot, making the missing bits for the engine, no matter what. If any naysayers should make negative comments, pay no attention, just keep trying and ask questions if you aren't sure.
I'm not certain what castings or other parts might still be available for this particular locomotive or if there were any other castings or parts included with it. You might try a post here asking if anyone has plans or if there were articles in Model Engineer or other publications with plans and text, or "words and music" as LBSC used to call them. Bear in mind, with some practice, you CAN fabricate any part of the engine if castings are not available or are unaffordable.
Is the boiler complete with all internal plates and tubes, and is it soldered yet? I see there are no holes or fittings in the top centre of the boiler. It's almost better these days if it is not yet soldered – if the copper is good, and the right thicknesses, you can probably work with a club boiler inspector to guide you through building a safe boiler with good materials.
Good luck with it. JD