If the difference between the Gripne and Gripen NG is not fan art---then why wishing it for the JF17 be something outrageous---.
The JF 17 from its inception was originally desinged to take the chinese engine---the WS10 or the WS13----. The JF 17 was NEVER built around an engine---but was given a modular design to accept engines in the similar category and range.
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For an aircraft like the JF17---a modification of increase in a proportional size of around 10-20 % is no miracle out of the skies but good aeronautical engineering---.
The smaller the aircraft---the easier to give it a 10-20 % increase in wing size---larger the aircraft---more difficult it is to increase its---dimensions---.
The fuselage is not changed by that much margin---a 2-3 % increase in size would be more than enough.
Technically---building a two seater from a single seater aircraft maybe more challenging.
But the important part is moving the wheels outwards and having taller wheels---. Same thing---if you put taller wheel struts---the outer wheels would have to move farther out proportionally to keep the aircraft's stability.
It is not rocket science---.
Honestly and now I might sound a bit arrogant, but that is plain stupid:
First of all the Gripen and Gripen NG both use the same engine - at least from the same family - which makes it possible to use the standard internal dimensions for the engine bay.
If the JF-17+ would use a similar modified or uprated engine, it would surely be possible, but what You are suggesting is to use a GE F110 or PW F100 as used in the F-16 for the Gripen and that would only be possible by a complete redesign of the fuselage to fit the wider engine.
Did You ever compare the RD-93 vs the WS-10's engine diameter. They both simply do not fit without a major redesign ... which in return would require a larger intake due to different airflow, airmass ...
As such, Your statement is simply plain wrong, ... the JF-17 was never designed to use a WS-10. The WS-13 for sure, since it is an RD-33/-93-development, but surely not the WS-10. Maybe You are mixing both.
In consequence You seem to take it too easy ... it is surely possible to enlarge the diameter, to increase the size of this and that and to get in the end a more or less new aircraft. This however is more challenging than to omit a certain avionics box or tank and add a second cockpit under a new canopy. Just look on most twin-seaters: they are all more or less the same for the rest of the aircraft other than the cockpit-compartment.
The same for taller gear-struts and larger bays ...
Again, it is not impossible ... but surely not that easy and by the way.
If it would be so easy, why is this not done all the way ?? All aircrfat are usualyl designed in a certain way to fit a certain requirement and to a certain budget ... neither the F-16 nor the F-15, were simply enlarged by a few meters or percent. The only aircraft that probably fits this way is the development from the F/A-18 Hornet to the Super-Hornet even if even here they retained a similar engine. But You are proposing is the development of a completele new aircraft that even more is de facto a J-10 with a slightly different configuration. And by the way, who should pay for this all ??
I really sometimes get the feeling that some here think China is willing to pay for all. For each and every new item they cry out "PAF want a few ! "Will we get them?" (just look at the J-10-thread) ... but they simply forget to be realistic.
Just my 2 cents.