State of the road
The road from Banyuwangi to Sempol is awesome.
The road is like new. There is tarmac all way to the entrance of the Kawa Ijen (national?) parc, there are very few potholes on the road (if any).
The reason for this, is that there is now a bicycle race called Tour de Banyuwangi Ijen, running every year since 2012 (or 2013?) so the road must be able to welcome racing bicycle!
Riding during the day
Extremely easy. Just take your time and you will reach the entrance of the parc in about 1h30min.
Enjoy the ride, it's a nice one, the road is great and the country side beautiful.
Riding during the night
Completely doable, it will just take a bit longer than if you do it during the day. You will also miss the view, but you'll have it on the way back I suppose.
I did this partly at night (the last third of the journey), and the only thing is that it gets pretty cold! It gets quite high, so cold too, make sure you bring some quality clothing (thermal clothes, and a jacket).
Renting a scooter in Banyuwangi
Extremely easy. Just contact one of the local motorbike renting companies and get yourself ready to go. I got in touch with one called Tripoli Tour & Travel, I booked the scooter while in the train to Banyuwangi and it took 5 minutes to sort the paperwork when I came to their office. Was perfect.
Here are their details: Tripoli Tour and Travel, Jalan Basuki Rahmat No. 77, Banyuwangi, Jawa Timur 68414, Indonesia, http://goo.gl/maps/EAncY