Model 7 engine rebuild

When you say stronger,do you mean more power?. If so the main components needed will be crank,barrells pistons pushrods and carb. The barrells are the most scarce but you may be able to entice another Norton man to swop his spare good dynamo  barrells for your m7 parts.To make the motor more robust (stronger) you will have to consider some new aftermarket con rods.  My preference  would be to "hop up" the model 7 engine with an alloy head ,better cam etc. 

The performance of the 500cc engine fitted to the M7 (and Domintor 88) was quite adequate for the M7's road holding ability.   Why thrash a 67 year old bike?  Modifications will be costly, and ( if the bike is in anyway near original condition) reduce it's value.

Should you decide to alter your machine to 600cc, it will have some more power and torque. Not a lot though. However, the 600 engine is not as robust as the 500 and is more likely to blow up under hard use. 

Just enjoy the bike for what it is, an elderly 500 with perfectly adequate performance.