Sorry for the late reply.

Thanks Max for the info, and spring location smile.

I had a look at the key, measurements are;

15.3mm tall
3.6mm thick
6.5mm wide

Based on those measurements (+/- for errors), I'd be looking at something like 4 x 6.5 x 16 mm (https://www.ebay.com.au/itm/183813241781?hash=item2acc1f7bb5:g:1xQAAOSw68Rc28Lf) ... though it does say hardened steel.

I did a quick search for a parts manual for the mower but could only see the user manual. Does anyone have an idea what clone this engine is from?

The key from ODK is certainly priced well, but I can't see the measurements, and the postage is a killer.