I've built scores of single speed conversions and about 10 with 24" wheels. My preference is to use aluminum rims from 24" bso mountain bikes. They are all 36 spoke hole count in that product range so the rims will fit most old school hubs. 24" bsos are mostly free around here. $5 tops. Look for one with good rims. Those chinese aluminum rims are "acceptable". Probably the only usable part on those bikes. the target age is 10 to 14 years old. Most get outgrown before they wear out. Or because they ride so bad, they deter kids from riding them at all. Bottom feeder brakes, gears and suspension make for a *&$% ride. Rim size is 24 x decimal point so there is a huge variety of tires to fit in colors, widths, tread patterns. the decimal sized rims are usually smaller in diameter than the fractional sizes so you get more room for bigger tires. Scuff them up with sand paper and you can paint them any color. Light weight rims will improve the ride of any bike.
Any bike I've done with 24" fractional sizes, well, I toss the original rims and go decimal.