Does the engine actually turn over, or does the solenoid just click-click-click? If it does, install a relay - use the 'start' trigger wire to switch the relay, and get the relay to feed battery power to the solenoid. (you could test this by feeding power straight to the 'start' terminal on the starter motor)As for aligning your spark plugs, that is really getting serious to get the last 1/10 of a kW. And really, it can only be done with the head off, because you are trying to line the plug electrodes up so that they send the spark into the middle of the combustion chamber.
Just bung in the new plugs (I'm using NGK PFR6B-11 (platinum), gapped to 1 mm), and NGK recommend 20 ft-lb tension (and that's not a lot of tension) in a cold engine.