I have a home made one I will send you a pic of tonight. I used a pit bike intake and adapted it for a 16 or 19MM (I cannot remember) Mikuni style carburetor. I have both pieces!..BUT! There are two different mounting patterns on a Briggs 3HP. If you are using the older style, it has two holes and my manifold wont work. Mine will work on the later style, 3 hole mounting. Can you take a pic of the motor?
...and I also have an old school carb/tank (I wanna say they are the Vacu-Jet?) from a 2HP that will fit yours...tank is clean...talk to me Sista!