The first test is to check voltage at the starter, min 9.5v cranking, if it's less there's a problem in connections or starter. Remove spark plugs and retest, if it's anywhere close to 9.5v with plugs out the starter is worn out (if all conn's are clean and tight). Whats the code #? First 2 digits are the year the engine was made, I sell a lot more opposed twin starters than anything else, same size armature for twice the CI.
B & S recommends a min 230CCA battery.
The starters should free spin about 9500 RPM's, it you have or can get a vibro-tach that's another way to see how they perform.
DO NOT GET THIS STARTER REBUILT!!!!!!!!!!!!!
The armatures wear and most rebuilders don't replace the armature, the cheaper, less than $100, price point starters are cheap Chinese pieces of crap, go with OEM B&S, or with Outdoor Power Equipment, OPE brand, the only Chinese built one I've seen that's half way decent.