I believe you're misled. The reason two strokes aren't as popular as they once were has ABSOLUTELY NOTHING to do with emissions or any other hokey story somebody tries to sell you.
The rise of 4-strokes in competitive offroad racing is a direct result of the AMA allowing 450cc 4-strokes to race 250cc 2-strokes, and allowing 250cc 4-strokes to race 125cc 2-strokes. This act has almost killed the 2-smoker.
OP: Riding on tracks is a ton of fun. So if I were in your boat, I'd definitely spring for a motocross bike. Now the 2-stroke vs. 4-stroke thing is up to you, but I'll hit you with some facts.
4-strokes have a more smooth power delivery. They fire every other stroke, so the power is less abrupt. They shine on slick or hard pack surfaces because of their ease to control. However, on the track, they're line selection is a little limited. They like wide arcing turns that you carry speed through, where a 2-stroke can come in hard, bury the front end, slide the back around, grab a little clutch and be back up to speed. Also, 4-strokes are a lot heavier and a LOT more expensive to maintain. You have to periodically have the valves adjusted, and when its time for a top-end rebuild, you'll be out at least $1000.
2-strokes are light, flickable, have a very snappy powerband, and are an absolute blast to ride. They do take a little more effort to ride, though. If the rider makes a mistake, things seem to happen a little more quickly. But 2-strokes aren't limited as far as line choices on the track. They can cut out of a line at anytime, which is an advantage when racing against 4-strokes. cc to cc, they make a lot more power and torque. A 250 2-stroke makes 10 more horsepower and 10 more ftlbs of torque than a 250f.
The biggest selling point for me is the cost. A two stroke top end can be rebuilt 3-4 times for the same price as one 4 stroke rebuild. I just got the latest issue of Motocross Action yesterday and they had an article about a YZ250 (two stroke) that had catastrophic failure due to being run too lean (not something that happens often) and the head and cylinder needed to be repaired, and it needed a new piston. They send the cylinder and head out for repair, did the rebuild work themselves (because its easy enough a monkey could do it) and it cost less than $500 for a completely new top end. That's about a fourth of what it would have cost to rebuild a fourstroke.
Do some browsing at
www.motocrossactionmag.com. they have a lot of throrough bike tests and they don't sugarcoat anything. Let me know if you have anymore questions. Feel free to PM me