Can't be done easily.... Look... trails and hair scrambles are one thing 100mph is another... I know what your talking about when you say you have an open field where you and your buddies have pissing matches... think we all do... but unless they are rockin something serious you should handle them... build a 4mill, get it ported grab some 28mm pwk (personaly I'm for them), pipes run your gearing closer to 14 so you can still hit the woods and they won't stand a chance... now if you want more build a 421 serval or something...
when you guys are racing each other... if you take off and are 12 bike lengths ahead how long do they really keep trying lol you don't need 100 out there