I was given an abandoned bobber project and it didn’t come with front brake caliper. I’m trying to do this the least expensive way possible. Can I use aftermarket brake calipers?

I bought some on eBay but the bolt holes don’t match up. I wonder if I can just add a custom bracket?

Front brake caliper brackets are easy.
1- tape a piece of 14 gauge insulated electric wire to the outside of the rotor. that will be your clearance.
2- set the caliper where you want it. clamp or tape to stop it from moving.
3- make a pattern from a piece of cardboard
4- transfer the cardboard to a piece of 3/16'' steel.
5- cut the steel out, drill your holes, and clean it up with a file.
6- remove the wire, bolt it up, check your clearnces, spin the wheel. no grinding sound means your all set.
7- take it back apart, clean it up with a wire wheel on a drill motor, and paint.
you will want to use all metal lock nuts as the caliper can get too warm for nylon

try to find a left side caliper from a SV650, they are dual piston and even with the stock master cylnder and rotor you will have much more brake.