first thing to consider is is there a bike already done that looks like what you want? chances are it'll be cheap to buy it and maybe just repaint it to what you want and polish the shiny bits, nice stickers (see ima)
if you find some longer forks, chances are you'd also need the wheel, fittings, brake caliper, mudguard as yours may not fit, also the back wheel might not match, you'd need the fittings, brakes for that to, then you need a way to attach the brake somehow

seats tend to only fit the bike they are ment for, but you can get them reshaped, and recoved (not cheap)
if you've got the time its a fun thing to do but it wouldn't be a weekend thing unless your very lucky, do honda do a CB200 custom with longer forks? (i don't have much to do with chops), but as suzi and ima say it can be hard work too! things can go wrong and thet always do