How can you fly your own military jet in terms of altitude and acrobatics?
But practically can you fly it?

Sure.

I mean how high can you climb?

As high as you want, assuming you have an oxygen delivery system (14 CFR 91.211, note that &quot;civil aircraft&quot; means &quot;any aircraft not a public aircraft&quot; means &quot;any aircraft not owned by the government,&quot; so you can't get out of it that way) and assuming both you and the plane are certified for IFR flight (14 CFR 91.135).

How fast can you go (I know no supersonic flights are allowed above land, but what about sea)?

Below 10,000 MSL no faster than 250 KT, and below a Bravo shelf or near a Charlie/Delta airport no faster than 200 KT, unless the minimum safe airspeed for the aircraft is higher than that (14 CFR 91.117). For supersonic flight see 14 CFR 91.817, 91.819, 91.821.

Can you do barrel rolls and pull Gs?

Assuming the aircraft is certified for aerobatic flight (or perhaps if it has an experimental certification), yes, as long as you comply with the restrictions in 14 CFR 91.303; or failing that, submit an application for a waiver of Part 91 rules (FAA Form 7711-2) (see also) and get such a waiver.

Also, can you use it to fly internationally (assuming that you can attach external fuel tanks in the first place)?

Yes, if you comply with each country's respective airspace/border entry rules.

Can you actually have fun with it lol? Assume that money is no issue.

Depends what you think is fun.
Basically there are no special rules for ex-military aircraft; if you can legally get your hands on one, and if you can legally fly it, you can do whatever any other aircraft is allowed to do as long as you follow the rules any other aircraft has to follow.