Tall paul,
The J21 also had an ejection seat and a Daimler Benz 605 liquid engine. And it had cooling problems. 297 planes were built between 1945 and 1949. The engine was also built in Sweden. And many modifications were needed.
And you are right the C.G is a problem. Fortunately the wing is swept. Otherwise it would be even worse. I have calculated that 20 oz will be needed. That is 10% of total weight. Not good but acceptable I think.
The J21 is seldom seen as scale model. And that is not without reasons. And then I have not mentioned the landing gears. They are sooooooooooo long.
A feasibility study would say no. But it is a challenge. And after having built RC models for 30 years I am glad this kind of stuff exists.