You have to do it all.
1. Night hours: You need 3 minimum.
2. FAA exam: 60 questions done on a computer and you must have a 70% pass rate.
3. FAA medical: Class 3 is all you need.
4. Oral: Do not underestimate this. The DPE (examiner) will not go easy on you just because you have a JAA license. The oral often takes up to 2 hrs, so be prepared.
5. Cross country. You must have at least 5 hrs of cross country as defined by the FAA: 50nm. In many countries, like in the UK, the cross country is slightly undefined and anything away from your home field is pretty much a cross country flight. So you need to have some longer flights in there. Most of my old Swedish XC's were defined as 25nm back in the days, so they didn't count either. Luckily, I had enough other ones.
5: Checkride.
Also plan on prob 15-20hrs instruction to get used to the US regs, depending on how busy the area you fly in is and how out of touch you are with power on stalls, turns around a point and all that stuff you probably haven't done in a while. I'm up to 20 hrs myself now, but then I fly in the LA basin out of Santa Monica, so it's a pretty high workload in this airspace.
But not to put you off - flying in the US is very rewarding. There's over 450 airfields in California alone and similar numbers can be found in other states - it really is GA friendly and supportive when compared to Europe. So see your training as an adventure rather than a stressed goal to get the license as fast as possible.