Used 1 tank full of regular gas - replaced with mix but won't start - what should I do?

Are you sure this is a DT 125 as with this particular bike it is self mixing there should be a separate tank by the handle bars just under the front plastics opposite the water tank this is where you fill it with oil then just put gas in the gas tank and it will mix it self you do not have to do anything else do not put oil in the gas tank with the gas mixing would usally be dirt bikes or really old bikes only

If it wont turn over, it may be because the cylinder is full of fuel, remove the spark plug under the top cover, with the plug out does it no crank over, if so give it a good few pulls to clear it out, replace the pluf and try again, using no choke and full throttle, just one point you didnt fill the fuel tank with oil by mistake??

depending on how long he ran it there may be internal damage to the engine. if you have not ran it i would suggest draining the tank and replacing with the proper mix. start the blower and see how it act. if he did do any damage you will know as soon as you start it. if you are not able to get it started the the rings may be junk.

50:1 mix in the gas tank and bar oil in the oil tank. Fill your oil when you fill the gas. Keep the chain sharp.

You are going to need more information before you start this up.
You need to know whether it is 2-stroke or 4-stroke.
You need to know the manufacturer, I see some use 25 parts gasoline to 1 part 2-stroke oil. Other machines use a different proportion.
If it is this manufacturer, mix say 25 ounces of gas with 1 ounce of 2-stroke oil in a separate container and pour into gas tank.
If it is 4-stroke, you will need oil in the crankcase, not in the fuel tank.
Try to find the name of the bike/quad somewhere.

Is it 2 cycle or 4 cycle? Meaning, does it take plain gas, or mixed gas?

Clean the plug, clean the carb with either choke cleaner (4 cycle) or wd-40 (2 cycle) by spraying it into the choke hole after you take off the air filter.

You may also want to spray some of the choke cleaner or wd-40 into the gas tank with the gas to help clean out the whole fuel system.

Only use choke cleaner for 4-cycle regular gas units. For mixed gas, use wd-40 in place of choke cleaner.

Try to start it and then go back and forth from half choke to full choke until it starts picking up.

I had the same issue with a Husqvarna 150 BT blower. Brand new ran through 2 tanks of gas and need to buy more gas , mixed 50 to 1 ratio 87 octane on new gas which was the same as what i was running and it started pinging and running like **** then stalled and would not start. took the plug out and it was fouled. cleaned the plug got it running again but still not good. This model comes with a NGK CMS7H plug which i replaced with a champion RZ7C and new gas 93 octane and lawnboy oil mixed 50 to 1 ration and now its running like a champ. I've had bad experiences with NGK spark plugs in the past and thought this may resolve the issue and it did. Hope this is helpful to anyone out there having the same issues. Also used some fuel stabilizer in the fuel mix, I hear this helps with the ethanol problem in the fall/winter gas.

regular or unleaded are fine if it is a four stroke engine but if it is a two stroke engine you will need to fill an oil tank also with two stroke oil, or you may have to mix it in a can , see manufacturer for mix ratio.