First: If you’re adding a lens/mount/etc to a holga, You’re Doing It Wrong. Second: the cost of adding a lens mount, buying lenses, working out all of that will likely equal the cost of a low-end used Medium Format camera (which I think is what you’re looking for). My suggestions for used Medium Formats: – Mamiya RB67 (6×7 format, lots of lenses and equipment available, totally manual—no batteries) – Pentax M645 (6×4.5 format, also lots of lenses available, again totally manual) Both are available as used kits in the $300—$400 range If that really isn’t the route you want to go, you can look at cutting out the current lens, and applying a new mount. You’d need to use “step down” metering for the aperature—that is, you’d need to manually reduce the aperture on the lens before you took each shot, but otherwise you’d be pretty good.