If I miss the flight, can I get a replacement ticket at Hangzhou airport

If you miss the flight, you may get a replacement ticket, but different airline companies have different policies. So you need to consult the airline company.
You take the airport shuttle bus catch the train, I think you can catch it. The shuttle bus takes about 50 minutes.
if you miss the train, you may change the ticket at the railway station.
You can collect your tickets at the ticket windows or the ticket machines.