They do make custom exhaust manifolds but there's already plenty off the shelf ones available

For inlets there's only really cosworth or a replica, unless you're lucky enough to find a mountune or go the difficult route and adapt a rwd layout caterham one (also fairly hard to come by). None of those are cheap either, pumaspeed cosworth replica is the cheapest at around £400, cosworth are almost £1000 and mountune ones are going for over £1000 if you can even find one.

What has always put me off the cosworth/pumaspeed ones is the short runners optimised for high engine speed but losing low and mid range torque on a NA engine.