I assume you've got a DHCPv4 server on the CHR end of that tunnel to assign an IP to the clients connecting via L2TP. If so, why not
set up a parallel DHCPv6 server so the client gets both types of IPs?
There are
a bunch of different IPv6 transition mechanisms, but I don't know if RouterOS exposes what the underlying Linux kernel offers in any way. These days, it's usually better to go dual-stack, as with my DHCP suggestion.