Short and simple answer to your question is, YES, there are numerous ultralight AIRCRAFT that are flying with 18hp rated engines (and less - hp) and have been for years.
I understand from your questions and comments that you are completely new to this aspect of flying, and it is understandable that you will have questions; you would be foolish not to (have questions).
At the same time do you really want to fly or just kick tires and dream about it ?
Most of the people getting into this FACET of aviation will research the various offerings available and most will purchase maybe two or three plan sets to better understand what comprises a viable aircraft; I own ten PLANS for the following - Proven Aircraft: Bower's Headwind, Evans Vp-1 Volksplane, Sorrell SNS-2 Guppy, the Sky Pup or Mach None, and the Beaujon ENDURO, as well as the Chotia Woodhopper; also have plans for the Mitchell Wing, Easy Riser and Quicksilver (weightshift) gliders that were both built and flown, as gliders only. Oh yeah almost forgot my Bensen Gyro-Gider, a total blast !
I settled on building the SNS-2 Guppy with a 1972 750cc BMW engine since it is enclosed and I am too old to enjoy freezing my butt off in an open frame type ultralight. THE BMW engine was chosen because it is more than reliable and I own two of them, A No
Brainer, plus I don't have to cut down a VW two get half as much HP as the BMW produces.
At this point in time I hope you have come to understand that the purpose of an ultralight is SIMPLY to get into the air, get out of the pattern and fully enjoy
FLYING in a machine that truly permits you to be a part of the experience, same
Reason people ride motorcycles instead of driving a car - to experience your surroundings to the fullest.
Forget all the mathmatic mumble-jumbo such as as nose/tail moment ratios and WING loading, etc., etc AND find out which ones have a GOOD support system still available.
Then and only then pick the one you believe will suit your interest, and build it - to the plans, If you want to fly it more than ONCE !
Best of Luck. LW