The Holga is 645 format from the factory, but it has the switchable red window for 6x6. All you have to do is to make a 6x6 mask. The lens, however, does not provide enough coverage for 6x6 without causing vignetting in the corners. Using the camera as 6x6 by removing the 645 mask entirely and not replacing it is why many people have light leaks. The mask is part of the whole system which keeps the film flat and tight, and which keeps out light. Also, just making a 6x6 mask by cutting out the extra material in the original 645 mask and using just the rim is not enough. You have to fabricate the inside part of the mask with paper or something to block out light from inside parts of the camera. Another thing that causes apparent light leaks is from (1) people not being used to loading and unloading 120 film and inadvertently letting light get into it, or (2) loose winding of the film, which causes light to leak onto parts of the film when unloading. This can be remedied too, if it happens.
I've given a lot of thought to how I would graft a different lens on that camera, like from a 6x6 folder. I've thought of taking the lens from an old cheap Polaroid pack camera. But, really, what would be the point? The camera IS the lens. If you already have a 6x6 folder, why not just leave the lens on it and use it instead of a Holga, or if you have an old, cheap Polaroid pack camera, why not just use IT as a toy camera, since pack film is readily available. Of course, the perspective would be different, because the Holga has about a 60mm lens, and 6x6 folders generally have a 75 or 80mm lens. One idea that might have some merit is somehow using the same lens (to keep that Holga look), but grafting on a shutter that allows a range of shutter speeds. Then again, my feeling is why not just use a camera that already has shutter speed settings. Part of the appeal of the Holga is making it work within the severe limitations it imposes.
On the other hand, with a spare Holga, it might be fun to make modifications like that, even though, it really wouldn't be a Holga anymore... but it could simply be satisfying to make such a customization work. I think it wouldn't be too hard to graft a folder shutter/lens combination on there (probably using the existing lens barrel and some epoxy glue, and then to deternine focus points using a ground glass.
Personally, I think an old box camera would be a much better platform for such modifications, but then, you wouldn't be able to say it's a Holga photo :)