First off, SNMP Traps can be divided into SNMP Trap fields. You can enable/disable SNMP Trap Fields from (File -> Setup -> Inputs -> SNMP).
Secondly, I don't see any email counters that identifies SNMP Trap fields by default. All email counters can be seen here. Hence, if you are going to email a SNMP Trap message, you can only use %MsgText counter and not SNMP Trap fields. In other words all SNMP fields enabled will be put in %MsgText counter.
So to answer your question, what I would do is to just enable SNMP Enterprise and Message fields then send this message through email.
Now, if you wanted to enable All SNMP Trap Fields and just email SNMP Enterprise and Message fields, this might be available by running script which will put the fields into VarCustomXX. But I will be honest with you, I haven't tried it since birth , but I am very interested to try it too. Let's wait for other Kiwi experts if they have other thoughts.
Hope it helps.