Not knowing the mission your plane is intended for, and weighing 100lbs (as you have stated) I would go with nothing less than two 100cc engines. You did mention twin??
Now your choices, by my minimum standard, would be 3W 106 or BME 115. Flip a coin If you are not well versed in gas engines maybe try a DA100. They are a little more forgiving for a newbie.

BME 115 new Mejz 28x10-6650+ (stock muffler) Both weigh the same as the 80cc engines you have mentioned
BME 110 new Mejz 28x10-6400+ (stock muffler)
The BME 110 is no longer in production.

If you want the most accurate thrust/hp calculator going, find the one Pe Rivers has made. Nothing comes close to it, and it is made in a spread sheet format. Free Too