BMX Bike Sizing and Changing Parts?
20.5" is way to big. You likely want something more around 18.5".

There are a number of bmx bike sizing charts out on the net to help guide you. Two examples are:


https://www.usabmx.com/tracks/1709/pages/492
https://www.danscomp.com/serve/products%7Ccharts/riderchart.htm


Other items you'll want to consider changing in addition to the frame size are going to be crank length, stem length, bar rise and bar width. Assuming you bought a pre-built bike, likely they are all oversized (for you) on your current bike. You may even be better off with a different wheel size.

If you were to get a smaller frame and move parts over you would want to make sure the new frame used all the same sized parts. Specifically you would want to make sure the new frame used the same type/size of bottom bracket, headset, seat post, dropout and wheel.  

Do you have access to a local bike shop that has someone knowledgeable about BMX racing on staff? If so it might be worth stopping by and having a conversation with them.