It sounds like your downhill forks might require a 1 1/8" threadless headset.
Slip a 1" threadless stem over the fork steering tube. If it is too tight, you need a 1 1/8".
A bike shop may determine the size for you for free.
My mongoose Zuma is 1 1/8". Others are Specialized shark, KHS Brentwood, Trek Clyde, GT also made one but I can't recall the model name. Nashbar made one.
I also have a Diamondback Voyager that is a mountain bike, but has curved tubes in the front triangle like a cruiser.
I don't think that any of the above are still produced, so you will need to check swap meets, craigslist, EBay, etc.
The Giant Simple is till in production. The various Felt cruisers also have 1 1/8" head tubes.