Will not start or fire new plug fitted - Garden

First, check starter assembly whether it's catching properly.
No spark usually means problem is around the spark plug -> spark plug boot -> lead wire -> ignition module + flywheel. Make sure you have the proper gauge and tester to check this. Like a ZAT4 spark tester, for example.
1.) Weak or no spark: Check whether you have the right spark plug and whether the gap is right. Requires a gauge.
2.) Weak or no spark: Check the gap between the ignition module and the flywheel whether it is according to manufacturer specs . Requires a gauge.
Note: If you trace the lead wire from the spark plug boot, the other end is the ignition module - it sits close on top of a flywheel. Color is black with two "legs" extending at the bottom with the ends cut to an arc/semi-circle to accomodate the side of the flywheel. The flywheel is that round thick aluminum disk with ridges that resemble fan/fins.
3.) No spark: Check the lead wire and the spark plug boot. They might be corroded or have lost their functional integrity over time. They are inexpensive and are easy to replace.
4.) No spark: Time to replace the ignition module. They can be costly. Upwards of $45USD. Plus you really might need a technician to properly seat and gap it.