Heres what I'm using:

Aluminium hub with 5mm counterbore 2mm deep (for the straight sided section) radially. I use dia 5mm depth 2mm neo magnets and stick them in with epoxy, holding them in place with tape over the top while the glue dries. The idea is the magnet sits flush with the diameter of the alu ring. You could just use a drill to make the 5mm bore as a poited end to the hole is no problem, it just fills with epoxy.

If its a hall pickup the gap between the magnet and the pickup should be 2mm max, the smaller the better.

Also check as some hall pickups only work with a N or a S, some work with the magnet either way round. Just try moving the magnet past the sensor first to make sure you get it in the right way round!

You ported the Poulan yet...?

Cheers,
Rich