My current bike is a 1981 Honda CM400. It is great fun and plenty peppy. It gets me around 60 mpg and my commute is 7 miles that is mainly 45 mph with some lights. Getting 100 mpg would be very difficult without a good amount of modification. So, IMO you really are going to need lots of mods and something smaller than a 400.
If I were looking to replace it with anything I'd really prefer to go electric. However, thats probably cost prohibitive (even diy isn't cheap). So, alternatively I'd probably go with something a bit smaller engine wise with fuel injections. IMO carbs are a big pain.