If headshake was not previously an issue, in order of likelihood:
Front/rear Alignment (Improperly aligned after chain adjustement, or dropped/damaged)
Steering Head/Triples Too Tight or misaligned
Mismatched Front/rear tires (you didn't say if you had replaced the rear and not the front)
Steering head bearings worn/dimpled
Wheel Bearings worn
These are the most likely scenarios, please note that the first three require nothing more than a little checking to fix. Most are easily done for little cash, including bearings. Tire wear/mismatch would be the most expensive. My guess would be a combination of worn front, mismatched rear and a little steering head wear, and mebbe some tire pressure issues. A little bit of each can throw the whole ballgame off. Fix most and the problem may go away; unless the front is badly cupped.