What is your primary goal with using 12" travel? Get more ground clearance? A more heavy duty handling fork? Ability to handle heavy load?
Reason I ask is that choosing 12" of travel will add a weight penalty. It could even be very large. Todays DH forks are about 3 kilos give or take. A used dirtbike fork might be upwards of 10+ kilos more with wheel, brakes, calipers etc.
Before deciding to go for moto cross fork, ask yourself if there is other ways to reach your goal. For instance, weight ability can be encountered by added pressure in air forks combined with stiffer spring in the other leg.
If it is more ground clearance maybe a bigger rim can take you there?
I have an old thread about mx forks, and the ones I asked in thread and PM's about their first hand experiences with mx forks all said in retrospect that mx fork was not worth the effort and added weight. So finally I too had to "settle" for DH fork. For me it was weight issues I feared to cramp the DH fork. New spring fixed that.