First question is what model of bullet because there are a few bikes now which have been marketed as a "bullet"?
On a more personal note, I'm interested to know more about your project and why you'd want a longer swingarm on a bullet? Pure aesthetics? Of all the bikes I've ridden, I wouldn't have said a bullet is drastically unstable in terms of road holding, has twitchy handling handling or has trouble putting power down without wheelying. Those being the usual reasons I'd think of someone wanting to do this. Or are you making an awesome stretched-out hillclimber for Red Marley?
If it's for aesthetics and for road use, you will probably also need new shocks too to push the rake angle back out again or it'll flop over into corners, and stronger springs to counteract the extra leverage.
I suppose with a standard one being basically two tubes, if you can't find something to fit, it wouldn't be the worlds hardest one for a decent engineer to cut, slug and weld a bit into a standard one. Maybe with some added bracing/gusset/both because 4" longer is a fair bit.
You might struggle to find an interceptor swingarm.