Re: BMX-90cc-four speed-trail bike
Worksman has 20" wheels with their clincher rims, drum brakes, and 11g spokes.
back in the 70's/80's you could get dimpled Femco rims with HD spokes, before ultra-lightness was important on BMX bikes. i don't think you can get anything like that anymore.
but you could always look into getting moped rims and tires.