Re: 1982 Mercury 18 hp Swivel Bracket Assembly
18 and 25 are the same motor just ported differently of this vintage. Not sure how much newer 25 swivel bracket will interchange.
Few thoughts:
-check ebay for 18 or 25 in this year range early 80's
-check your local Mercury repair guy shop (the one with a mountain of parts in the back too good to throw away, ask around to locate)
-find a 25 with a blown powerhead and missing the lower unit and install your powerhead and lower unit.
good luck and lots of 25's out there that will likely work.