thats a bad way for putting it. lets assume they both weigh the same [1500kg] and the cj-10 engine's more efficient. the issues that would need to be addressed would be actually fitting, cg and software updates.
obtaining the enignes i dont know but i doubt. even if you cant get it i doubt pakistan is full of dummies who cant even assemble a team to develop a good engine. its like you said, if we [they] cant buy it, then we build it.
cruise missiles which weigh about 1500kg have engines with thrusts varying from 400-600kgf. 450 kgf being typical and on the tomahawk. id imagine Pakistan should have resources in Belarus, Ukraine, china, France, south africa etc..... to obtain not even the engines but the designs for a fuel efficient engine. as for the structure. id imagine pakistan would have gone for aluminium alloy whilst the newer missiles are composite.
in regards to a previous post you mentioned a weapon which is classified, id imagine it would be a short range slbm as typical sub manufacturers are beginning to develop vls systems in 11meters lengths. mind you a short range slbm is mealy a step into the realm of larger missiles.
supersonic missiles are not the way forward to an extent. look at india, which would you consider more of a threat, the brahmos or the nirbhay? for me it's the latter. i expect you to know why.
my point is i doubt the engine is an issue, the range is played down too much in my opinion.