After going to major airshows like Oshkosh & Sun N Fun since the early 90’s the one ultralight that really stood out to me as a “solid aircraft” is the Aerolite 103. The design is extremely clean with only the bare basics needed for flight.
Brand new fully complete fly away cost are $20 – 25K (circa 2019)
Kits minus engine start at under $14K (circa 2019)
The web site for the 103 is:
The main distributor is located in DeLand Florida @ KDED
Engine options range from 30 HP Kawasaki’s to 50 HP Hirth’s
at one point Hobby King produced an electric powered RC model of the aerolite 103 cleverly called the aerosport 103.
Model aviation magazine did a write up in 2015:
You may be able to find one on eBay. With the open frame structure this would be tricky to scratch build as an RC aircraft