It has a 69mm crank with 87 mm P&C - so 1641 cc. That extra 1 cc counts.
Check the hinges for the front hood and rear engine lid. If it is an IM, they will have INTERMECCANICA stamped (raised casting) in them -- if they are plain then it is a CMC/FF. A top and windows will cost $1k. Built in early 2000's on a 1965 VW chassis with only 200 miles - sounds more like a CMC. I have one sitting in my wine cave! Also check to see if painted or OEM CMC gel coat.
I'd think 70-75 HP would be wishful thinking over the stock 1584 cc 50 hp engine (69 x 85.5) - referred to as a 1600. It would need some good heads, cam and exhaust. Even fiberglass, it still weighs 1800 pounds.