Flavored Brandy to Fortify Port

I have used orange brandy to fortify a orange chocolate port. I have also used a ruby port and added chocolate and blueberries to tweak. Both turned out very good. I would stay with a similar taste as the original port style is flavoured unless it is a ruby then you have a neutral base to work from.

that depends on the port. I know a few wine makers that make ports from all sorts of berries and fruit and that would probably work well on a few of them.

Just remember that when making port, the "brandy" that is typically used, is about 95% ABV and is used to stop the fermentation by bring the ABV of the fermenting wine up to about 20%. That brandy used is neutral, and is more like moonshine or everclear than anything else.

Use brandy in port only as a flavouring tool, not to boost the ABV, because you'd need too much to do that.