Painting it is the easy part.
You'll need to pull the engine out of the bike to get it done right though.
Once you've gotten that far you'll need to thoroughly clean and degrease the whole engine. Then his all the surfaces you want to paint with some 3m scotch brite and then degreaser once more.
Then you'll have to remove all the pieces off the engine not getting painted (i.e. carbs, throttle bodies, etc) and tape off anything else not getting painted.
I would also recommend pulling the timing, clutch, and cam covers and just having them powder coated as that will hold up much better than paint on those pieces.
Once you've got the motor all prepped and ready for paint find an engine paint and color you wish to use. The stock color triumph uses seems to be a matte/flat black. Duplicolor and VHT both have matte black and satin black (if you don't want totally flat) engine paints. I've used both on my Ducati and they have held up well.
Lightly coat all the surfaces you're painting to get a base layer down. (I like to hit each layer with a heat gun in between as the paint cures with heat and will cut down drying time). They lay down several layers (about 5 should do) hitting it with a bit of heat in between and letting it sit for a bit between coats.
You'll have to be very careful once it's all painted not to spill any fluids or chemicals on it though as the paint will not be cured and resistant to it until the engine as gotten hot and fully cured the paint.
Good luck with the project!
"It's not leaking oil, it's just sweating from all the horsepower!"
Mods:Arrow 2-1, BC FEK, BC indicators, BC Quick release cover screws, BC AIRK, CRG mirrors, LSL Rearsets, black 'chazzos', custom 'brushed finish' headlight, NH Damper.