For mods, just take everything out of the addons folder and put it somewhere where you can access it later so that you can transfer the files back when you re-install. As for savedata, I don't know exactly how SRB2 retains its data, but I know theres several .dat files in the main folder that could be a step in the right direction. I can't tell you if saving those would work, though. Maybe instead of uninstalling your current SRB2, just install another in a different folder? It's been a while since I've installed it, so IDK if you can do that though.