The old starter is a normal type, solonoid mounted on top that engages the bendix and drive gear,it has a firewall mounted solonoid. most of the aftermarket starters Jegs,Summit,Powermaster etc are gear driven, hense gear reduction ,which provides more cranking power they are also lighter in weight and most are adjustable as to positioning the starter. in the case of the Jegs starter it has a shim to move the starter fore and aft to correct any engagement issues
with the drive gear.
My comment as to the tooth count making no differance comes from a starter builder I bought a gear driven starter from years ago, and after replacing ring gears on a Tiger several times know you will never buy an exact replacement, as to the comment as to the Tiger having a different flywheel, I’m not sure that is correct I have used the Jegs starter on a 302 and now a 260 and have never had any issues the only issue is the clutch a Tiger having a long type while
others use a diaphragm type cover but thats another deal which is an easy fix!