J85s albeit small are old-tech turbojets, thus: fuel-guzzling. That's why I suggested modern turbofans. You said RB.199, and I said: J101.
That one would delightful for a few reasons a) it was the YF-17 engine and b) it was the F404 direct ancestor, so F-18 and F-20. In a sense, a J101 F-5 would bring together all the varied Northrop fighters from P.530 to F-18L. Don't forget Northrop got screwed thrice in a row: YF-17, F-18L and F-20. They ended truly squeezed between F-16 and F-18.
A case could be made that Northrop had two post- F-5 paths
-go single engine: but F-20 came too late
-go twin turbofan: YF-17 and F-18L went nowhere, to MDD delight.
...so how about going F-20 right off 1968 rather than 1983 ? just swap the F404 for its ancestor: the GE1 / J101. Put that on a F-5E.
That one would delightful for a few reasons a) it was the YF-17 engine and b) it was the F404 direct ancestor, so F-18 and F-20. In a sense, a J101 F-5 would bring together all the varied Northrop fighters from P.530 to F-18L. Don't forget Northrop got screwed thrice in a row: YF-17, F-18L and F-20. They ended truly squeezed between F-16 and F-18.
A case could be made that Northrop had two post- F-5 paths
-go single engine: but F-20 came too late
-go twin turbofan: YF-17 and F-18L went nowhere, to MDD delight.
...so how about going F-20 right off 1968 rather than 1983 ? just swap the F404 for its ancestor: the GE1 / J101. Put that on a F-5E.