So...
You're looking at an embargo tank, made mainly from US parts (why bother reinventing the wheel when you have plenty spares lying around?). Make that an Abrams or Patton!
Frankly, that's the most logical solution for the short term (until the spares run out). I think regular/NG armor bases and/or storages are still more numerous than qualified plants in the US, so any seceding state or group of states would have enough running tanks and spares to go for some years. They could also rush to capture some from hapless neighboring states.
The problem is, you can smelt everything you want, with decent foundries and heavy motor plants (making trucks, tractors, mining vehicles), you can probably come up with some old-style armor good enough to invade Connecticut or Mexico, but to replicate modern MBTs from scratch, you also need advanced technologies.
A 120mm smoothbore gun isn't exactly low-tech, nor is Chobham armor, not even mentioning the active parts (FC computers, stabilizers, thermal sights...).
I think you should look into what the Israelis came up with in their early years and post-embargo period, before they had the industry buildup to go Merkava-level (btw, consider that they took 30 years to come up with an indigenous tank). Also take a look at the Yugoslavian armor industry post-breakup, when the factories building the various parts had been taken by the different independent republics.
Now if you want a couple of homemade designs, let's get going!
