yeah don't really see the 18tm around anymore
so looks like the 21tm for you

you could also consider rb
they have a drop in for the revo also
tm523 comes with pipe header and engine for like 300
have heard people say its a screamer also
could always use the search tool and see what people say about it

could also consider the pico .26
would have to pick up their engine mount
could get that whole getup for like 200 bucks
i've ran it before and it was ok
front engine bearing leaked after a while for me
have heard a few other people say the same thing
seen others have slight tuning problems with it
it ran fine for me
kinda has mixed reviews really
i know some people who just love it
and some people who would never buy it again

just thought i'd give you a bit more options
hope this helps you a bit more