It is essentially a bolt on. But, remember that there are 2 types of aluminum swingarms that Ducati made for the 92-98 900SS and 900SS/SP bikes. There was an update to the aluminum swingarm circa 94 (in the US) to strengthen the top bracing that runs from the sides of the swingarm up to the shock mounting bolt. I'd stick to the updated ones due to strength.

I dunno how many supposed used 900 SS/SP bikes, which only came out from 94-97, I have seen with the older aluminum swingarm.

You can tell which is which because the updated aluminum swingarms have a thicker cross section on the top bracing. Comparing pics from a known 92-93 to a known 94-97 will make the difference evident.