Customize Email Tokens

You certainly can do this with a Velocity ("Email Script") token -- and so much more it will blow your mind. You need to search the Community for "Velocity" and also read the seminal posts at https://blog.teknkl.com/tag/velocity (while Velocity is a standard template language not unique to Marketo, these are the only Marketo-specific deep dives in existence).

In this particular case the Velocity equivalent is:

Dear ##
#if( !$lead.Salutation.isEmpty() )
${lead.Salutation} ##
#else
Dr. ##
#end
${lead.LastName}

But you should familiarize yourself with the wider world of Velocity rather than concentrating on only this one need.

Unfortunately there isn't as in-depth of a guide for Velocity as there is for AMP. Check here for starters though: developers.marketo.com/email-scripting/