I don't see a topology picture associated with your post. But the short answer to your question is to look at the configuration guides for PSTN access for SRST hosts and simply change the target of the dial-peers (that would normally be pointing to the PSTN) and have those dial-peers point to your SIP gateway. If the incoming calls during an SRST event would also ingress via the old SIP gateway, the inbound "PSTN" dial-peers would also point to the old SIP gateway.
Essentially, look for a configuration example for SIP PSTN support for SRST (Google is your friend with that) and change out the PSTN SIP target in the example for your old SIP gateway.
Give it a shot and post a draft here and we can help you tidy it up.
Maren