some additional information would be helpful:
- the full model number, usually found on a decal in the cavity under the tilting seat;
- the make/ model of the tractor's engine.
-reason for replacing the original starter.. and source of replacement starter( OEM or aftermarket)

Above info will help those experts that regularly answer many of these posts...

Any number of things might be keeping the starter from operating.
Did you reconnect the battery cables correctly? Did you get the starter mounted right? A starter that didn't get bolted in place correctly, will make it "jam" when trying to start the engine. does the starter gear look like the old one? Did you get it on backwards?
If the old gear looks ok, it can be reused. If you disconnected both battery cables, did you get them back on correctly?
If any of this helps, please let us know! Don't just go off into the sunset, leaving us wondering if it helped! RJ

John Deere Z425 zero-turn mower with 23-hp Briggs & Stratton.
Turning the key produces nothing more than a click and no cranking, even with a fully charged new battery and freshly wire-brushed terminals and lead connectors. Brake is on and steering handles are stowed outward for correct microswitch positions.
Relay? Wiring? Leads?
Any insight would be much appreciated.
Thanks.