You may be as well off using the standard one tbh, you need to make a lot of alterations in there so see how it goes. If you have a plastic manifold any more than 5 or 6 psi could rip it to bits tho, but standard engine internals may not like a lot more.
There is a guy called Sev who makes alloy custom parts near to me, he makes intercoolers, manifolds, what ever you want really, if you get stuck he could make a manifold to fit.
the manifolds on the m44 are cast metal, there are superchargers available (KO performance i think) that utilise the stock manifold w/ minor modifications but they are sold as kits and are expensive. i'd love to find a cheaper alternative to do to my 318ti aswell, keep updated
Looked into this lots for a different project and several people have ended up making their own, obviously needs a fair bit of skill / tools.
Several books on Forced Injection that enable you to calculate the capacity to get good flow to each cylinder but so far I'm not aware of anyone offering off the shelf ones for the 4 cyl models unless as part of pricey kits.
Mini Supercharger Eaton is well liked but gives a different curve on torque to Bigger Rotrex ones, still quite effective.
Seen this done on M42 a few times but not M44, although very similar of course.
speak to A-tech (www.fastbimmer.co.uk) i believe it a service they can provide, be ready to pay them about £2K and provide you own eaton supercharger, if the want rotrex about £3.5K, if you speak to them they may make you the neccessary parts to fit yourself? if you find a cheaper way of doing it yourself, please please please, let me know, i think i'm going to go down the turbo route purely because if i could afford an extra £2 - £3.5K in one lump sum, i could have got an 325/8 with change (enought to cover the extra insurance)
I have recently found a second hand downing atlanta supercharger on craigslist in washington (i think, it on my work email) i can't afford it, the guy wants a grand (£) for it, if anyone wants it i can post the link