Anyone knows how to fix the old engine has been sitting for decades? I got an expensive .21 engine but all internal parts has been dried up so the crankshaft can't be turned at all. Any clue?

First ,take it apart if you can ,if not take a heat gun an heat up the block this should help , then get a can of nitro clean, an clean all the parts, then take some afterun oil an lube everything, let it soak if need be.

I have done this and mineral spirits will clean it up .

I pirsonaly would not heat the engine i would defenatly try some solvents first just be very cearfull if the crank has the red puddy in it mitneral sprits will not efect the puddy but some stronger solvents will. If the crank has no red puddy in it id use somthing more agressive, i would defenatly try to disamble the engine first if posable and leve the piston liner and conrod soke in some solvent. Just be cearfull with of the crank if there is red puddy in it do not use any kind of spray cleaners it will attack the puddy. good luck Regards Jeremy.

One more thing to think about. If after cleaning and oiling the bearings don't feel smooth they will need replaced.

soak it in kerosene for a few hours- possibly overnight. it wont harm any of the seals, o-rings or the putty in the crank.

Bolt a flywheel on it and try to loosen it very easily.