I've been fitting shutters to Holga for about 5 years now. I'm making and making them to order here and in Japan.
They retain all the Holga focus, aperture and vignetting characteristics. Simply put, they give the same result a Holga does. Only, you mostly get shutter speeds from 1s to 1/500s, B and T. And a flash sync at all speeds.
Just start by measuring everything (film plane, infinity focus length etc) with callipers. Rip off the Holga shutter/lens mount and cut off the screw on mount. Attach that to the new shutter and attach it to the camera body. Screw the lens onto the mount and off you go.
That simple... not exactly. Lots of cutting, bonding, cutting, testing correct focus... the usual Holga modding heaven n hell. Just keep an eye on keeping measurements true.
It still throws the ppl in the pro lab into fits every time I pick up a dozen rolls of perfectly exposed Velvia from Holga.
I keep 2 myself so theres always one for Velvia and one for Polaroid. I even have a Pin-hole lens (40mm), which can be used on them. I've currently 3 orders to fill, so it'll be a while before any more would be available.
modified HOLGA.jpg

Thats a totally fake 70-200mm mod, btw. But still, a project in the making?