I had something very similar happen earlier this year on my Evo. Sheared the keyway on the pinion shaft, threw the cam and breather out of timing, front exhaust valve was crashing into the piston and sounded like a bell ringing and kept puking the dipstick out.
Ended up redoing my entire top end and went with new cylinders, Wiseco pistons and headwork through Jim Gutches out of S.C.
A 3 dollar keyway cost me bunches................just sayin'............hope this ain't what happened to you but man, you describe what was happening to me to a Tee.
Pull the plugs, and remove the rocker box uppers. Rotate the motor in 5th gear with the rear wheel off the ground and watch for the valve timing, intakes opening as the pistons are beginning to go down and exhaust valves opening when the pistons are rising. Use a long plastic straw in the plug holes to reference the piston movements. Mine was almost 180 degrees out when if finally let loose and I knew right away where to look. In the cam chest.