Bought a 1971 Sportster 1000 XLCH that only had a kick starter so I bought a starter assembly off of eBay, clutch basket with the ring gear, front gear, modified the primary case to accommodate the starter assembly and bought a bigger battery. I can get the starter gear to engage in the ring gear manually but when I try to start it with the ignition switch it will turn but not engage in the ring gear to start the motor. Any suggestions on what I'm doing wrong or need to change..??

 

Haven't been to the Sportster side for a while.
Don't know if you got it sorted out, but sometimes people put the brushes in incorrectly after a rebuild of the starter and this cause the starter to rotate the opposite way, which could be the wrong way.
Starter motor needs to turn clockwise, to turn the engine counterclockwise which allows the bendix gear to engage properly. It's beveled as such to allow ease of engagement.

 

Could be the starter solenoid is not throwing the starter motor out into the ring gear.