AMT did indeed put out several versions of the Havoc in 1/48 scale several years ago. I remember a A-20B/C, A-20G and even a P-70 night fighter. They were nice kits, although not up to the Hasegawa/Tamiya standards of today. In 1/72 scale things are worse. There's ancient B/C models by Airfix and Revell and a G model by Matchbox (molds now owned by Revell) that's not too good either. Try looking at model shows or ebay. I don't expect any of these kits would be expensive today.
Regards,
Pat