When I try to start the small square I think relay in fuse box buzzes loud. Then no engine start for a few seconds ? Have to hold key in start position. Also do not know if this is related but PTO will not engage blades/
So the small relay is a start relay. They vary by build date so for more specific info you may want to provide the serial so we're all on the same page. Not a big failure item. You could be looking at some funky connections at the relay, fuses, battery or elsewhere. Know what you are starting with power wise and have the battery load tested. You like to see 250 CCA or greater on the load. Seeing as though you have a PTO issue, start there.
You will want to know that you are getting 12 volts to the PTO clutch or not. A basic multimeter set on DC will get the job done. No power and you've covered the bases as Dave noted - concentrate on the connector at the rear of the switches remembering about the switch also at the park switch as the PTO won't run in park.