Hi guys

I would like to upgrade a differential on my classic, but to be honest I have no ida what needs to be changed or how much would it cost.
Basically I am looking for better driveability(By that I mean powerslides ,donuts)
Because I don't know what diff I have right now, but the car is very understeery and almost impossible to send sideways.
I am planing to build a proper track day car so I would really prefer if it didn't understeer that much.
Thanks for advice in advance ;-)

Martin