x2 on air leak. buy starter fluid and spray it around the crankcase area while it is running. it will most likely be by the clutch mount on the crank or the magneto mount on the other end, so if you spray starter fluid in that area and notice the engine revving up, that is where your problem is. fixing it involves taking the engine out, taking cylinders off, pistons, then splitting the crank in the worst case scenario. best case scenario is you take the engine out, and find a shaft seal that is serviceable from the outside, ei just take off the magneto and seeing a bloken seal you can get out and replace with only moderate difficulty.
if you blew a seal on the magneto side ive often noticed magnetos get gas and oil and them and then fail so you should keep an eye on that aswell. good luck