There are two reasons to use lights: one is to see with; the other is to make you visible to others.
Strictly speaking, the lights should be attached to the vehicle, not the rider, to comply with UK law, but as so many cyclists use no light at all, the police are very unlikely to worry about lights that exist but are not fully compliant - especially if the rider is on a bloody great unicycle with no brakes.
I have owned 3 head torches of very different design, and all of them have been adjustable to fit round my helmet. I would be surprised if yours cannot be made to fit.
The problem is getting the elastic to sit still instead of slipping up the outside of the helmet. An answer to this is to get some sticky-backed Velcro discs - the hooky ones, not the furry ones, and stick them to the helmet. This will snag the elastic of the head band of the torch and hold it in place.
A 3 LED head torch allows enough light for steady road or pavement riding, and easy towpaths and the like. As it is near to your eyeline, there is some back scatter from fog or dust, and it also attracts insects towards your eyes, which is a Bad Thing.
Any single point of light will give you only limited information about the surface, because you don’t get the full 3 dimensionel effect. The ideal is to have two sources of light, although very powerful head torches (£100 plus!) will do the business. I haven’t ridden in the dark for a while (getting old and lazy), but when I did, I relied on a 3 LED head torch and carried a small hand torch (Mini Mag-Lite). I’d use the hand torch for scanning the route ahead, and giving more detail on tricky bits, and the head torch to provide a pool of light a few metres ahead of me. It is important to be able to ride by feel, so that you use the lights to tell you what’s coming, but your skill to deal with what’s immediately under your wheel.
As for the lights to be seen with: there are various cheap and cheerful LED lights available. I have a red LEd mounted on the seat post, fairly high up, and a white LED mounted just above the fork crown.
Good luck, you’ll have lots of fun.