hello there my friend, guess what I have just wrapped up for post tomorrow
(i finally found them!!!)
now I believe the problem as you say is the RS232 (serial protocol) to PS2( TTL protocol) conversion.
here is the problems
PS2 operates at 5v / Serial operates 9-10v
PS2 & Serial use different protocols
but this i believe you have already discovered this.
I might have a spare serial mouse, although, its been a while since i have seen one.... the last one I remember was made by Kingston.
eBay is your best bet however.....