I think its time for a strip down.
Take the motor apart - its really not that complicated. This will give you a chance to check out the valves, guides and exhaust threads on the head; the state of the pistons and rings, and the bore clearance in the cylinders; split the case and check the cam and state of the crank.
If the engine was re-done, the head and cylinders won't cost anything.
This will give you a chance to check out the crank - perhaps it wasn't assembled properly. Check the crank for runout and the condition of the journals. While at it, have the crank, rods, pistons and rings dynamically balanced. 80-84% seems to work well in a P11.
Best case scenario, couple hundred bucks for the balance and gasket set, and you'll know the engine is right.
If the crank is bent, any Atlas crank will be a straight swap, and you could use a 750 Commando crank since you'll be re-balancing anyway.
If you find the head was never done, or the cylinders are worn, bearings shot, etc., it could all be done for under a grand if you do all the assembly yourself. If you replace the bearings, use a superblend on the drive side and a ball bearing on the timing side.