Well, there is no conversion kit for a SBC, mainly because it’s not a very desirable swap in the Supra. It has been done, but you can find more 500+ hp 7M-GTE swaps out there vs SBC.
If you intend to put a SBC in it, why even bother messing with all the stuff on the 7M? Since it is not a daily driver then you can pull the 7M out and find yourself a nice built SBC and get it done by spring. That will save you time in the long run.
On a side note, the 7M-GE is pretty darn efficient for what it is and the technology it was designed with. Its nothing like a SBC where just a mild cam and headder gives you 75+ hp, it is almost at its peak in its stock form, even stripping all the non esencial parts on a stock 7M is fruitless, nothing will be noticable gained. And like said previously, things on it are there for a reason.