Hi Adam,
An excellent question, but unfortunately a bit open-ended. So here's some questions to (hopefully) envoke substantive input to your question, and narrow the bandwidth on this thread.
a. Where is the boat located? On the hard in the winter? Afloat at a full-service marina and/or boatyard? On a private dock somewhere? Close to home?
b. What is your level of do-it-yourself (DIY) comfort? If you're planning a DIY replacement, you're going to have to dig into many systems aboard (furniture, electrical, plumbing, etc.) to replace your tanks.
c. More importantly, what is your level of DIY capabilities? Given your initial query, I expect you haven't tackled a project like this before. And it's a BIG one! Arguably the biggest project a boat owner can face during his ownership.
d. How long do you want to own this boat?
e. What are your usage history and plans for this boat? Marina hopping? Multi-day off-grid trips?
f. Have you engaged your insurance company and/or a surveyor to determine the extent of your fuel tank issue(s)?
g. What is the size of your wallet? Replacement of fuel tank(s) is typically a VERY expensive proposition, at least to do it correctly. And if you're planning on having professionals do this work (see b. above), then your original posting hopefully will lead you to these professionals.
Unfortunately, your original posting may evoke many, many responses that will be less-than helpful. In particular, the "howler monkeys" may chime in and post advice to "pour some mouse milk into the existing tanks. It'll work just fine." Or others that suggest rote replacement of large tankage with multiple small (usually plastic) fuel tanks. And without knowing intimate details of your particular situation, or intimate details of the OA 50 Mk 1, much of this advice may be fruitless. Given I have absolutely no knowledge of either, anything I may add at this point will be equally fruitless. Other than to say "been there, done that" on multiple large powerboats in the past.
Best of luck in this venture. And kudos for posting a search for "cost effective" vs "cheapest" solutions to this issue. They very definitely aren't equivalent. And "cost effective" and "DIY" are also non-equivalent, depending on who's asking.
Regards,
Pete