There are a number of ways you could cast/replace the missing pieces. But yes, I'll focus on the cheaper ways, if you dont want to be spending lots on Silicone and resin...
1. There is a substance called Insta-mold (there may be other brands)... eg
http://www.thetrolltrader.com/catalog/p ... nt_mold/70This can be heated in hot water and pressed over the wheel, to make a mould and then use the mould to make the missing wheel from pressed in greenstuff.
2. You could use Air drying clay, which is very inexpensive to make a clay mould, just lubricate the wheel very lightly with some kitchen oil or petroleum oil (eg vaseline) to enable the wheel to release easily without deforming the soft clay. if you let the clay go hard you *may* not be able to remove the wheel without destroying the mould. Then again when the mould is hard fill and make a wheel with pressed in Greenstuff.
Both these methods would only copy the detail on one side of the wheel and edges. Which is fine as the other side will be hidden under the catapault.
The throwing arm is trickier.
1. use the instamold as above just on the throwning 'spoon' and weight. Completely surround the spoon or weight. Then when set, carefully cut the mould in two with a sharp knife and fill each half with green stuff and press them together to set. Use some balsa wood or plastic to make the actual wooden arm and fasten the moulded pieces you made to it.
2. Make the whole throwing arm from scratch. I one built a whole Isengard ballista from sheets of Balsa wood, and a sharp knife and some glue. In this case again make the wooden arm from wood or plastic, sculpt a rough rock from GS for the weight (or GS some iron 'bands' over a real stone you find), and find a round plastic cap or lid that might do for the spoon to hold the ammunition.
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