You would have a gravity of around 1.075+/- depending on the specific extracts and exactly how much was removed from the Munton's can. You might want to add a small amount of bittering hop, perhaps 1/2oz of something in the 4-6 alpha range, but it wouldn't be a requirement. If you like a malty, sweeter Porter leave it as is, if you want it a bit more balanced go with the 1/2 oz... Either way, it's a beer I would like a taste of
If you happen to have a smart phone, there are a number of free apps out there that will do the sort of thing you want.