I havent personally, but from what i understand quite a few bikes have been built this way, in the 60s and 70s especially. Ed Roth's Oink! being one notable example. I dont see why it should be any different or harder than stripping down a normal bagger, most police bikes ive seen (harleys at least) are pretty standard dual-shock swingarm cycles with off the shelf parts. Surplus property auctions for state and county owned equipment and vehicles go on all the time too, check the classifieds in your paper.