For security reason, I have to forward UDP 500 / TCP 4500 and ESP 50 to a secure network in my internal network where a VPN device manage a L2L vpn for this secure network. Unfortunatly I cannot use the meraki MX to manage this L2L vpn.
Re: VPN site to site port 50 forwarding protocol ESP
You won't be able to forward ESP. I suppose you could try a 1:1 NAT and set the forward rule to ANY... But I haven't tried this so I don't know if it would work, and you're effectively opening the whole box to the Internet, which isn't a great idea really.
Bascially you're going to have ensure you have NAT-T enabled on your VPN links. You won't be able to do this without NAT-T.