Chainsaw Repair

There's a good chance the flywheel key has sheared off and the flywheel moved out of position changing the timing. You will need to pull the flywheel and see if the key (likely cast into the flywheel) has sheared off.

If it has, make sure there is no oil in the flywheel bore or on the crankshaft. Line the keyway in the crankshaft up with the flywheel sheared key and tighten it down to get the timing back where it should be. I believe the torque on the flywheel nut (If it's an M8, is 12 - 14 ft lbs). It will run fine without the key. If you feel you have to have a key in there you will have to replace the flywheel.
The saw may have a separate key which you can replace.

Make sure the hub of the flywheel is not fractured or c-r-a-c-ked (that word edited out by a prudish checker) the flywheel could come apart under speed with dangerous results.

Fossil can I buy the flywheel? Reckon what it may cost?

Fossil can you set the air gap between flywheel and coil? What setting if you might know. The flywheel really appears to me to be ok thought air gap might be off throwing timing off!

Thanks will check in the morning.

I just place a business card between the flywheel magnets and the coil.  Loosen the coil mounting bolts and let the magnets draw the coil to the flywheel.  Tighten the coil bolts and you should be good to go on that end.