I have a couple 100cc rotary valve FA air cooled engines. I also have a water cooled 100cc reed valve. They are all DD.
The speed and sound will blow your socks off and the most fun you can have in a kart. The sound alone will stop people at the track to see what you are driving.
Two years ago, the F100 Series in So Cal planned to offer this class as their top class. It’s still in the plans, but may be a year or two.
In the meantime, I’d be glad to sell you one or two of these engines if your interested. I have some rolling chassis for $500-$1500 I could ship you, but you should be able to find some nice ones in your area.
You’ll have more luck finding an “ICA” rather than Formula A in the US. ICA was more popular, based on the same 100cc, direct drive concept with a few provisions depending on the exact years you are looking at. Basically ICA is within 95-98% peformance of an FA motor and still gives the same great driving experience that FA offers with broad powerband, high revs and a light kart.
You might catch an unmolested example on bobs4cycle once in a while.
Otherwise if you are looking importing, on facebook lookup the F100 Buy and Sell group for aircooled and the “proper karting 100cc” group for water.