People want to think that manufacturers are making 50kph bikes but restricting them to 25kph and if you just find the right kit to unlock ... nope. If a bike is sold to conform to X countries speed limit there isn't likely much more power or speed than that available. To go fast offroad takes LOTS of power and the bike has to be plenty strong to boot. You don't find that kind of performance at budget friendly price points. Yes, a BBSHD tucked up under a suitable Downhill MTB should do what the o.p. wants. Budget ~$1500 for motor + battery and ?~$700 to $1000+ or more for a bike. That is what it costs to go fast off-road. YMMV.