Help!

Hi, just joined and looking for help. I bought an old craftsman riding mower for $20 from a guy who couldn't get it started. I cleaned the carb and changed the battery and voila, new riding mower. It cut well for a few months and I let it sit during the cold season. I tried to start it again and I saw white smoke and oil coming out of my muffler, but no start. I replaced the head gasket, spark plug, oil, and cleaned the carb again, but still no start. No oil in the muffler anymore, but I just don't get any fire.

What do I do? I don't know if I'm losing compression elsewhere or how to ground the spark plug wire to check if I'm getting spark. Is it possible a valve is bad??

I don't want to take it to a repair shop as the lead time is 3 weeks this time of year..

The engine is Briggs and Stratton 289707-0154-01.

Any ideas?