the jag engines if kept with good oil pressure and clean ish water are very reliable....if you lose oil pressure stop immediately and investigate.
I would, if it is possible fit an inline oil cooler to add more oil, and help with the cooling by fitting some additional piping to an adjacent water cooler, maybe one situated outside, maybe....would certainly improve reliability in my opinion, if the head overheats warping is the name of the game and a real pain...skimming works sometimes but many times not.
I would also consider having the head ported to give it better flow through and improve its power delivery so reducing the engine revs for a given power level...dont port for revs port for low down torque...
I have owned many jags and only ever had one problem, i drove one with low oil pressure...silly me buggered the engine...total rebuild needed, in winter in pouring rain, but ran beautifully after that...