If the MiG 19 fuselage diameters followed the Area Rule, instead of being a constant size cigar, perhaps transonic performance will be improved; someone answered to this question elsewhere indicating the airplane is already somehow area ruled, from the swept back wing, the place of canopy, but the B-58 'Hustler' fuselage is actually 'Area Ruled', you can have a look at it. Think in some USA aircraft cases where Area ruling greatly improved results.
Maybe a little, but not really worth doing.
Area ruling is not a precise science. The exact cross-section used to calculate the area depends on the Mach number, so the best you can do is pick some problem speed and area-rule for that. Also, junctions and other features do not always blend smoothly, so the ideal area profile as you work along the plane is not achievable. You end up making compromises.
And even then, unless your design already has a problem, area ruling will not magically push your plane faster. For example the English Electric Lightning had a similar straight-through fuselage design yet was capable of Mach 2 plus.
The MiG-19 was one of the fastest "pre-area rule" types in large part because it had among the fewest problems with sharp changes in area. The kind of re-contouring that Convair undertook between the YF-102 and YF-102A would not need to be nearly so drastic on the MiG-19 and would not bring anything like the same benefits.
The Mig-21 is in some ways what you suggest, a developed MiG-19 with area ruling (and a few other little tweaks!). Prototypes were originally built with swept and delta wings, and the delta only then selected for production.