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so ive been fixing up a 1974 yamaha DT125, its a 1 cylinder 2-stroke (although gas and oil are seperate and mixed in the carberator) with 4-stroke oil used in the engine.
in the process i have had to replace the piston with a brand new one and re-bore and fix the cylinder and such
anyway i was curious if there was anything special i should do when firing it up for the first time.
i know when you buy a new bike they say not to exceed "x" amount of RPM's and i didnt know if anything like that applied to my situation

thanks

The best thing you can do is ditch the motor oil in the injection bottle and use dedicated 2-stroke injection oil. You can find it at most places that sell a selection of oils like auto parts stores or Walmart-type stores as well as motorcycle shops.

When you first start it up, let idle until decently warm then just ride it for a few dozen miles at a pretty relaxed pace. Don't rev the snot out of it, but always have it pulling a load or decell-ing fairly hard. You want to avoid long, steady, single rpm operation.

Don't let it overheat, if it seems to be running hot, you probably have to re-jet the carb to feed it more fuel. Your oversized engine may want more fuel now that it is sucking more air, it may not, depends on how closely it was jetted before the rebuild (among other things).

The rings will seat pretty fast and after a 100 miles or 2-3 hours running, it will be as broke-in as it needs to be.