Assuming you don't mean bits as in tunes for the engine, rather you want software to try different fenders, headlamps, etc., I would think the most inexpensive tool you could effectively play with would be Adobe Photoshop.
You can download a free evaluation copy of Photoshop
HERE to see if it meets your needs before buying a full version.
Take a look at some of the cool concept bikes Birdy68 has put together in the Thunderbird section using Photoshop:
http://www.triumphrat.net/cruiser-pi...ed-images.html
Check out the cool dual headlamp image and other nice concepts he's done there... enjoy!