Can I use BMX brakes on my Murray Ultraterrain MTB? I have an outdated 90's MTB that requires single pivot brake calipers. The only brakes of this kind I have found are either for road bikes or BMX bikes. How do I know if they would actually fit before my online purchase?
The features that need to match are reach from mounting bolt to rim and recessed / normal nut. A BMX sidepull brake will probably work, but it's better to check the measurements before buying.

For younger readers, Murray was a legendary BSO brand in the 1990s. They did come with centerpull brakes made out of thin sheet metal.