If it is a genuine Ducati oil filter then it will have a unique number of flats meaning you either need the correct Ducati spin on filer tool or if the Boa doesn't loosen it you can just about get a screw driver in it and butcher it to get it off. Do not try that method on fitting the new one
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Mr G is quite correct that the carb heater was available as a kit for the non cooler equipped 600's and personally I agree with him on them working well (once the oil is up to temperature) to eliminate the carb icing that can affect them in certain conditions.
As for belts I have used the ExactFit ones for the last couple of changes and extended the change times from two to three years as they recommend. I have found Stuart there to be super helpful and will give a discount if paying by PayPal..
Once you start getting into the resto properly it may also be prudent to check out the tensioner and fixed bearings for the timing belts as failure of those would undo any benefit of new belts.
There's also rebuild kits available for the vacuum pump from Allens Performance but if your 600 has a vacuum tap as well (like my 900) then that can't be rebuilt so replace that along with the vacuum hoses and fuel filter for the best performance of keeping the carbs full.