If the ROM is missing, I would think that some-ones removed it cos the machine was dead myself.
But if you want to spend money on that poor thing then.
I have a A500 PSU and some ROM chips kicking about.
BUT I also have another turd coloured A500 with PSU, SCART, MOUSE etc that i'm waiting for payment from a eBay user.
Total cost of it was £20 inc UK P&P.
And as I sold a PSU for £6 and a ROM for £5, you might as well buy a whole set-up

(look on eBay, theres LOADS of them)
There are also sellers who sell the bare cases (so your turd become a cream beauty again

)