Cant really help specific mods for your car, but try having a look for parts in the below site:
https://www.partsforvolvosonline.com/index.php?cPath=28
For modding your car, you are better of searching for the Volvo groups on Facebook, such as the below below
volvotuning
volvo600
volvot5owners
etc etc, there are tonnes of really knowladgeble people there all into modding cars.