I assume that the check running on Icinga 1 is not the same as the on used with Icinga 2. Am I correct there? You could just use the script from the Icinga 1 system, create a command for that in Icinga 2 and you are good to go.
Next assumption: check_command = "snmp" is the check_snmp script.
This script has these two options you can try:
"-l" = {
description = "Prefix label for output from plugin"
value = "$snmp_label$"
}
"-u" = {
description = "Units label(s) for output data (e.g., 'sec.')"
value = "$snmp_units$"
}