"Die", not tap. The threads are usually long enough for a regular freewheel. A machine shop would surely charge far more to lathe cut threads (you would have to remove all the spokes to fit any hub in a lathe) than just changing the hub. The thread spec is
1.375" x 24 tpi or 1 & 3/8" x 24 tpi. This is the same as english threaded bottom brackets. Many bike shops would have a tap but you need a die. I've never seen a 1& 3/8" die in any bike shop. A 1.25 for mtb forks would be the largest. Some early Gary Fisher bikes and some British tandems had 1 3/8" threaded steerers so there is a remote chance a really old school high end bike shop might have a fork threader in that size.
You could buy a die and handle and diy.
Here is a cheap one on amazon.
https://www.amazon.com/Aceteel-Roun...6b-a46e-f504c6411677&pd_rd_i=B09SY5CCXK&psc=1
If you ever cut threads with a die, the leading edge leaves a ramp which you sure don't want for a freewheel. All freewheel threaded hubs have the inboard end of the threads cut straight up to stop the freewheel from threading on further. As you pedal, the freewheel wants to keep threading on. Ramped threads would get it on so tight it would never be removable.
There are other freewheel thread standards. "French" and "Italian" but those should not be on a bmx wheel. Humm, the Italian company Campagnolo made some bmx stuff....
I have a GT bmx wheel that has a strange freewheel threading on the flip side. This wheel has 2 different size freewheels with different threading. Not a fixie/freewheel hub with a reverse threaded lock ring for the fixed gear side, it's a freewheel/freewheel hub. Just one of the many one-off designs in the bike industry.
There have been many 20" mountain bikes with freewheels. Most are heavy junk bikes so I often scrap them out. I have several 20" wheels with freewheels on them, usually 6 or 7 speed. There are some high end recumbent 20" wheels with freewheels too if you want something light strong and fast.
So what is the hub made of? Aluminum? Steel? Chrome plated steel? One of those ultra cheap box mart hubs made of thin sheet metal and painted? I expect most steel hubs have the threads rolled, not cut. Most steel hubs are made of several parts pressed together. Most aluminum hubs are cast blanks, then lathe turned with hardened steel bearing cups pressed in.
