I have a couple of Enfields, both in a trials style, one more so than the other.
The ride-ability of the bike very much depends on how "off road" you want it, as the better it is off road the less pleasant it is on tarmac.
Do you intend taking it off road / green laning or are you just after the off road style? Either way, the biggest styling change is the fitment of trials tyres and ideally you need to fit a 21" front wheel, as generally speaking trials tyres only come in 18" rears and 21" fronts.
You can get 350 x 19" trials tyres, which will fit both ends of the standard Bullet wheels, but personally I think that they look a little naff on the front.
I generally use Pirelli MT43 tyres, which are regarded as the road biased trials tyre of choice for long distance trials and these tyres perform well on the road and in the wet. (Other super soft 1-day trials tyres may not be road legal and don't last at all on tarmac.) I run mine at between 10-15 psi and don't have any grip or predictability issues on tarmac.
You also need to fit wider trials handlebars, which necessitates longer control cables, however this makes the bike a lot more controllable at low speed.
If you do intend taking your Bullet off road, you would ideally want the foot pegs a few inches further back, as this enables the bike to be ridden standing up and and enables you to take a bit of weight off the front end when bouncing over obstacles.
You may also want to gear the bike down to make it more controllable off road, you can get smaller gear box sprockets, but if you want to tackle anything challenging you would need to gear it down further.