Metro 5.5" Brake Booster Question

OK so i have a brake booster out of a earily 90s Geo Metro, the brake booster is roughly 5.5" across. Well i am using this booster in another application and was wondering if anyone knew where i could buy or if there was even a brake master cylender off another vehical that had a bigger bore on it?

The issue is the 5.5" booster is all i have room for but the master cylender doesnt move enough fluid for the brake system i have on the vehical.

Thanks for any information.

A 90 honda prelude MC has been swapped into a swift GT before. It's a much larger MC than the geo's.