I have a B&S 17.5hp tractor engine that when I disconnect the battery it shuts off. So I am guessing the electrical system won't charge battery. How do I fix the charging system?

I wouldn't remove battery for a test. I would start with a freshly charged battery, measure the voltage, start the engine, measure vlotage, if voltage is the same or lower, it is not charging. Could be a regulator if one is not built in.

What has been happening that would cause you to disconnect the battery. Was the battery always low? A fillu charged battery should read about 12.7 DCV before you start the engine. Tell us more about earlier issues

Sounds reasonable. If regulator is bad, is that easily replaced?

Motor was stalling after 30 minutes of use. After starting a few times and restalling more frequently I would have to charge the battery to get it to start / run again. I disconnected the battery to clean terminals / recharge. Tried running w/o batt