I think you mix up IPsec VPN and SSLVPN. The custom port looks like you want to use it with SSLVPN.
Yes, in FortiOS v7.0, there is a new option to use a custom port for the IKE and IPsec protocols when using NAT (default: ike/500, ipsec/4500). Both protocols will use the custom port. This might help if IPsec standard ports are blacklisted by the ISP, but if there is censorship, it will use whitelisting and thus port relocation won't help at all.
If you go this way, check that the VPN client can use the custom port.
"Kernel panic: Aiee, killing interrupt handler!"