sudo /sbin/ifconfig ham0 mtu 1200

#or

sudo ip link set mtu 1200 ham0

#persistence:

# create a file (example: hamachi) at /etc/network/if-up.d/hamachi
# with this contents:

#!/bin/sh

if [ "$IFACE" = "ham0" ]; then
    ifconfig ham0 mtu 1200
fi