Instax mini 30 + Olympus Zuiko 75/2.8
To mod the medium format lens to an Instax mini isn't that difficult. The hard thing is finding the proper lens with shutter for the mod. For Instax mini, the 75mm lens from 6x6/6x45 folder cameras probably is the most cost effective choice.
Due to the lens focus difference and mini's original lens barrel design, it still comes with a bit vignetting after all possible modifications. The color rendering tends to be bit yellowish compared with the original lens, especially with the indoor lighting. One shortcoming is that you'd better use a digital camera for light metering. The instant film is just as the reversal, not much latitude there.