You can use iptables SNAT, for example
iptables -t nat -A POSTROUTING -p icmp -d destination_ip_here -j SNAT --to-source source_ip_here
But few things you should known. Pings will not return to your host, but will or will not be delivered to your spoofed IP, it depends on security settings. If you use private IP range, routers will not route packets to hosts on public IP. Your router/provider can block spoofed source addresses and alert administrator about anomalies on network. You can be easily tracked down if you cause harm to other system.