It's a tough one, really. Given its issues, I'd say it's worth the sum of its parts, which is not an inconsiderable amount. Perhaps £150 in this condition? And, as I say, that's based on the parts.
I should point out that I would be interested in buying it if it were to be offered around this price. I don't say that in anyway to try and claim it but because I don't want there to be any suggestion of a conflict in interest in my valuation. Genuinely value at around £150 but, as always, it's open to debate so would be interested in seeing other's opinions.
*Edit* Just seen the missing piece you mention. It's the rear cannon and can be found readily:http://www.ebay.co.uk/itm/Vintage-Star- ... Swm7pZzUMP