Thanks net_sailor; I already use silicon paste to make some moulds, but I need some that would support filling up with resin. And at the moment, I only have GTS resin that I use for simulating water, but it turns into a clear and very stiff material when drying, probably not the best for casting spareparts.
I've taken down your advices and guess I'll have to do some experiments.
Der Adler; to make your own spareparts, you first need to make a master, that is a primary version of your parts in solid plastic. You need to cut and sand the master until it has the correct shape.
Then, it depends on what you need to do: for canopies, make a negative print of your master with modelling plaster, then vacuform the canopy: heat a sheet of clear acetate and press it between the master and negative mould.
For other spareparts: make a negative mould with silicon paste or rubber from your master, then fill the mould either with melted plastic (use sprue and trichlorethylen as a thinner) or with resin. But resin heats when drying so the mould needs to be heat-resistant while remaining flexible to ease un-moulding of the parts.