2-strokes run fine without mufflers, if your muffler pipe is an expansion chamber you might notice a low to medium power loss, but you should be fine.
Here's a little rant, but it's necessary.
Weather or not crassius is 100% right, or wrong for that matter, yes, cold a air on hot parts introduces a chance of heat shock, this chance is very thin, even pulling air back through the exhaust is very unlikely. What I think he's thinking of is the chance of particles, or foreign objects, like dirt or pebbles. These can cause damage, but unless your exhaust port is facing strait forward, and under the frame of your bike, than you should be fine.
Here is an actual rant, but it's helpful.
The glasses are still expanding as they leave the cylinder, meaning return isn't likely. Plus, when freshly fueled air is pulled into the engine from the carb, it's then pressurized by the crankcase pushing it into the cylinder. This air I speak of coming from the carb? It's 'cold', that doesn't damage anything, and the crankcase wouldn't have much of any time to 'warm it up' before it entered the cylinder. Does that damage anything? No. If cold air didn't flow through the crankcase, it would overheat, and s seize. As we all should know, cold air is denser hot air. The denser the air, the more oxygen atoms are present. The more oxygen you have the more fuel you must have mixed in to maintain a proper balence. The more oxygen and fuel you have, the bigger explosion you get. The bigger explosion you get, the more power you have.
Thank you,
Murkle