A 2TG could cost between $600 to $900 while an 18R is not worth the time it takes you to spit on it. I know, I used to have one.
Go the 2TG every every every every every time. Do not think about an 18R. Ever. Full stop. End of conversion.
If you want an 18RG, buy a RA23/28/40 first, then get this engine, after you drop the old 18R in the ocean to start a dive wreck or artifical reef.....