Can I buy the chassis only?
Normally, you can't, no. If you need it for a particular reason (like yours got broken etc.), I would suggest contacting a Baby Jogger distributor (or contact the Baby Jogger's customer service directly) and ask if they could order you the chassis as a spare part (arguing with your problem/a faulty frame - and the want to stay with the pushchair you already own). You probably will need to pay a fee for that, but it's your best bet.
Most distributors can order parts like these - the more so with the world-famous brands.