You are much better off withOUT that mechanic... he was really full of it...
If you replaced your own injectors... did you check their pop off pressures and pattern ? Did you put new washers to replace any one time use items?
An aerosol burst of starter fluid into the intake when starting one of our 617's won't hurt anything... not enough BTU's, and this has been discussed here LOTS... and JimSmith posted the final word...
Your problem is probably something minor and when you find it you will be amazed that you thought about giving up on it...and that that mechanic told you you needed a new engine...