Jaguar is/was a very good aircraft, IMO. Indeed the engine thrust was lacking, for all the things id was tasked to do.
(very toxic from the French to note that the navalized Jaguar was problematic in the engine-out situation, and then adopt the Super Etandard that had no engine-out abilities what so ever; I'm not counting the pilot ejecting as a solution to this problem)
The low-powered engines choice stemmed from the initial design being supposed to do just the training? Possible candidates to the engine replacement if it stays 2-engined are not that great. I'm partial to the RR making the turbofan spin-off from the Viper engine. A turbofan with t/w ratio of 7:1 was very much within the scope of any jet-making engine company of the late 1960s (design) /early 70s (production and use). Even just 6:1 would've been great.
Use the RB.199 as the parts donor?