Hey guys! 
I’m new to this forum and have a 1977 DT250 that was given to me I’m looking to rebuild. My father in law gave it to me and says it ran just fine until it was left sitting with gas in the carb. He says that it had piston slap as well and will need the top end rebuilt. I consider myself a fairly mechanically inclined person but have never taken on any kind of engine rebuild before and am hoping to find some sort of how-to video or set of instructions other than what I’ve been able to find on the service manual. Any info or advice would be greatly appreciated I’m looking forward to getting this beast back on the road. 

First suggestion. Buy a hand held impact driver. Second suggestion. Attempt all the work yourself first, take pictures, and post in this thread what you did along with your questions. Work until you get stumped. Come to this site with what you already tried on your own and the responses will be much more detailed from the awesome helpers here.

Hi Casper and   to the forum, from Andy in Ipswich U.K. One of the things we all like on this forum is pictures so please post some up of your bike.  It also helps the good folk on this forum to definitively ID your bike as the 'model' you think you have if they see some pictures too. Anyway good luck with the overhaul. Andy.

Greetings from Tennessee. 

I would get it running before you start breaking it down for a rebuild. And don't let him talk you into piston slap until you hear it for yourself. All my top ends make noise to varying degrees, and many people would consider it piston slap. But I just keep rolling along. 

Yep agree with SCHU above. Get her running & let's hear/see what ya got.
Hope the carburettor isn't too much of a mess--depending how long sitting. Drain some fresh fuel through by removing the bung under the bowl if its not too difficult. Then open the choke & leave the throttle closed untill it fires up & leave choke open untill it settles down & richens up. That's in a perfect world lol. Let's know what happens.

Appreciate the input I cleaned the carb and I’ll be putting it back on later today and hopefully it’ll be running. I’ll post some pictures as well of the bike. 

I do have another question. Turns out the oil pump isn’t working properly. Is there any reason why I couldn’t mix the gas in the fuel tank until the pump is fixed? I don’t see why not but I figured I’d ask. 

You might have to bleed the auto lube pump but you can do a premix of gas and two stroke oil, the ratio is somewhere on the forum. It is supposed to smoke. Might want to pull the head to make sure the piston rings are intact or you will need a top end rebuilt if they are stuck or broken. If I get a bike that's running it's a home run. Otherwise I dig in a little to avoid damage from running it.
Good luck

Update- put the carb back on and it’s leaking gas. I’m assuming the float is stuck so when I’m working on it tomorrow I’m going to try running some compressed air through it to hopefully free it up. As promised here is a picture of what I’m working with. It’s been painted and has a newer gas tank so it’s definitely not the average dt250 but I’m hoping to buy a gas tank and new plastics for it to make it look a little more original.