Gregor - I don't think you want to go down this path with this older system.
First, Yes, you can still find some older SSD's that have a PATA interface (used). So you could do that. The rub here is these are old tech and are on the small size so you will be limiting your system and have very little space to work with.
As to the OS you are using on this older system (OS-9 or older OS-X) does not have TRIM support so it will wear the SSD more quickly.
Given all of this it's still better replacing the HD with another one if your HD is going Vs a SSD.