The good thing about nagios style monitoring systems (I prefer Icinga2, personally), is that it is very simple to write your own plugins. The state is given be the exit code:
Output is simply output on the shell. So a simple
echo for bash would do.
To get perfdata you append a pipe and a space-seperated list of key value pairs to your output like so:
This is an example output | value1=1 value2=2
For checking SNMP you can use a shell command like
snmpwalk. If you don't want to write this from scratch I happen to have written a couple SNMP based checks: