Laying a bike down can often cause the carburetor to flow gas and flood the engine, which will make it very hard to restart. You might want to check all the overflow tubes on the carburetor and make sure that they are routed properly. The should essentiall lead down immediatly as they exit the carburetor. If one or more takes a round-about path it could cause the overflow to not over flow properly.
It also sounds like the jetting is basically in need of some work. Either that or you are not keeping it wound up enough. Two strokes have a definate power band and if you fall below a certain RPM they will just fall flat on their face, power wise. Two strokes will also tend to foul plugs if you don't run them hard enough to get the plug hot.
Starting going up a steep hill on a 125 can be a challenge. You may need to slip the clutch a lot to get going, then once you are moving you need to keep the engine way up in the RPM. You simply can not go up the hill slowly.
The first step would be to make sure that you simply aren't trying to ride a two stroke like it was a four stroke. Keep the RPMs high, let it scream.
Second step would be to check the temperature range of the plug. Get the bike running and warmed up. Put a brand new plug in it. Go for a ride, preferably long and representative of how you ride. At the end of the ride shut the bike down without an excessive amount of idling (Like ride in hot, shut down immediatly). Pull the plug and look at the color of the ceramic around the center electrode. Do a search here or google the internet to find "reading spark plugs" and compare the color of your plug. I suspect that you may need a hotter plug. Note: A plug is "hot" or "cold" based on how easily it can get rid of heat. A "cold" plug can transfer its heat more easily to the cylinder head and thus it runs cooler. A plug that doesn't run hot enough will not burn off the oil and other deposites and it will foul. A plug that runs too hot will self destruct and cause "pinging" and possibly engine damage. You can buy hotter or colder plugs that will fit your bike. For example, if you are currently running a NGK BR9ES a BR8ES will be an exact replacement that is just a bit hotter.
The next step is to settle on an oil mix. 50:1 is very much on the light oil side for a dirt bike. I run 40:1 (synthetic oil) in mine, many people run 30:1, and the manual specifies 24:1 if you are running regular oil. More oil will in general be better for the engine but worse for the environment. The oil mix will have a small impact on the jetting so pick a mix and stick with it.
Finally you will want to adjust the jetting. Do a search of this forum, it has been discussed many times.
As for a manual, go to just about any bike shop and look for an after market manual such as Clymer's. Most shops will have a reasonable selection and since your bike isn't that old I would expect them to have one. They should cost about $30 but they are worth having.
Rod