Welcome to the forum. 2 stroke China motors are most popular due to low cost and ease of installation.
There are always multiple trends here. Some are in frame tanks (like a modified Felt), recumbent trikes, four stroke motor kits, DIY builds with Briggs styled four strokes, Morini motors, rack mount motors with CVTs, shift (jackshaft) kits, ebikes with Lipo batteries, etc, etc. Checking out the photo gallery is usually enough to help you decide what you would like to get.