i wont talk about prime hiruzen, because theres no reference for his capacities , but the hiruzen we saw vs the kakashi were seeing now is also inconsistent because of power scaling, so to estimate what hiruzens abilities would have been in part 2 if he survived oro invasion is what needs to be done
given that, its hard to say who would have advantage taijutsu wise, weve seen how shinobi with experience like kakuzu can outpace kakashi even with sharingan active, so if were to say hiruzen gets a part 2 scale up he would have some energy to spare rather than fighting conservatively, id have to say hiruzen would most likely have the upper hand in hand to hand, although it wouldnt be too great of a margin because of the sharingan's analytic motion predicting abilities
in ninjutsu is where the complications arise, because as weve seen part 2 kakashi has a seemingly much deeper well of chakra to pull from, its hard to properly assess how much greater hiruzens chakra's would seem to be, but keep in mind hes only getting older and fading while kakashi was and is still growing as a shinobi, he would have an increase but it wouldnt be nearly to the scale that kakashi is pulling shit off currently to, 3 kamui's , something like 6-8 raikiris/lightning manipulations and a mess of fighting we didnt even see, and still ready to go some more, its impractical to say hiruzens chakras would get stronger to that extent when time is against him realistically. you could reference oonoki but he just seems to be a beast :shrug:
in terms of the actual ninjutsu use i believe kakashi has more counters for hiruzen then hiruzen has for kakashi, shiki fuijin aside, given their nature affinities. but hiruzen may have more, im not one to speculate though. we can only debate on what weve seen really[ im sure minato had more than ftg/rasengan/rds/8 trigram but we cant use speculation of his other abilities in discussion] when hiruzen combines his chakra manipulations will probably give kakashi some pressure though, in the end id have to say kakashi takes it with ninjutsu, not even counting kamui, which drains the tits off of kakashi's chakra
genjutsu is the one thing i think hype can be allowed on because when youre rumored to have mastered a 1000 genjutsu , even if youve only done half that, thats probably a feat never before seen in the shinobi world,[ besides that woman who stood with hashirama senju] kakashi hasnt really shown any serious genjutsu feats besides what the sharingan does to its opponents [ zabuza] so on hype alone hiruzen would take it in genjutsu skill clearly, would they be effective? who knows, im thinking visual genjutsu will probably be rendered null by the sharingan, if hiruzen had any sound genjutsu that would be the way to go.
in the end given my estimation on hiruzens part 2 power up and their fighting prowess, id have to say kakashi would probably take it high diff, but only with kamui, otherwise given hiruzen has a larger chakra supply to work with id say hed take it med-high diff