I have a mint in box optical mouse designed for Amiga. :-) It's called GoldenGate...something...works like a charm. No need for any conversion. Been thinking of eBaying it or selling it privately.
AFAIK, it was out before any MS PC optical, or similar mice. ;-)
I'd suggest getting one of those
MirooCheck devices from Elbox. You would be able to convert your Amiga mouse port to ps2 port, and then use a USB to ps2 converter on it if necessary as well, and you'd be ready for an optical USB mouse. ;-)
Another option would be to get a USB controller/ports for Amiga and a compatible stack, however not worth it IMO if the only thing you need is a mouse.