Graemes right although I use oxy (propane will do too)
Secure the barrel or head etc to a strong platform (a piece of wood in the vyce will do and a sash clamp or big G clamp to hold the item.
Position the lever under the affected fin (if it's a Maico it's a whole cylinder) and apply GENTLE pressure as you also apply the heat. The alloy will soften and at that point you will feel it yield to the pressure.
DON'T APPLY EXCESSIVE PRESSURE
DON'T APPLY EXCESSIVE HEAT
It's simple stuff
Dave Mac