The basic symbols in a list such as https://www.copypastecharacter.com/symbols should work, as they are ASCII characters and not reliant on extra encoding. These are a set of accepted text symbols that will universally work on all clients and many will convert them to emoji. Just highlight the symbol from https://www.copypastecharacter.com/symbols, copy and paste it into your email.
Alternately, you can work around this by q-encoding the emoji in UTF-8 format, which will render a string of code to add to your subject line. This code tells the email client to render the desired emoji. All you need to do is copy your desired emoji and run it through a Unicode to UTF-8 translation tool, such as https://tools.bluestatedigital.com/kb/subject-line-assistant.
As an example, say you want to send a Happy Birthday email to a lead. You could copy the (birthday cake) emoji and run it through a translator to yield the code "=?utf-8?Q?=F0=9F=8E=82?=" Entering this code in your subject line without the quotation marks will render the emoji on the client's end, and render it in the design most compatible with their device/browser. With any email design, it's always important to test before a live send to confirm it appears the way that you expect it to.