John T has made some good points but i would like to add a few more. There is a full range of aftermarket 3 point hitches avaialable for the JD letter series tractors. However, keep in mind that on your 46 A you have the older style drawbar frame and does not provide the support of the newer drawbar frames that were on the mid 1947 tractors and above. This will require that any aftermarket hitch you plan to use will need to only mount to the rear axles and not use the drawbar frame for support.
Secondly, be judicious in evaluating really how heavily built the aftermarket hitches are. There are some that are not built heavy enough, especially one that will only have the rear axles to attach to. Also look carefully at the concept used to reduce or eliminate sway. Most damaged hitches I have seen have been due to inadequate or none at all, sway protection.
An additional item to check is what bracketry is used for the Top Link Brace. First it needs to be very heavy but more importantly in the right position to create the right 3 point geometry for the implement in use. The better hitches offer multiple top link attachment positions. But equally important is verify that the bracket will not interfere with the hydraulic outlets on your Power-Trol. Many have in the past.
As John mentioned, also keep in mind that you will have a three point hitch compatability but will not have the features and capability of an "Active" top link, meaning no sensing of the load through the top link to provide draft and load control. This was the key feature of the patented Ferguson system and did not appear on JD tractors until the 1953 JD 40 and then later on the 20 series JD RC tractors in late 1956 for the model 1957 tractors.
Hope this helps a bit.