Gasket or no gasket, something's banging into something unless a bearing is gone ..
Since this started with a piston modification it seems reasonable that the piston has a problem or it is a part of the problem, or it's installation has revealed the engine has a problem elsewhere.
Get some thin, soft resin-core electrical solder. Bolt the engine together, including whatever gaskets are supposed to be there. Remove the spark plug and insert a strand of solder so it reaches the outside edge of the combustion chamber.
Crank the piston over beyond TDC, crushing the end of the solder. Remove the solder and measure it's crushed thickness. If it's less than about 0.02 inch (or half a millimeter) thick, things are too tight. Once the engine is up to running temperature this clearance will decrease, perhaps to less than zero.
Higher torque comes from a higher compression ratio. I'm guessing this "high torque" piston is probably taller than stock.. it's crown probably approaches the head closer than a stock piston would, reducing combustion chamber volume and so increasing compression ratio.
This modification may work just fine on an engine that's in good shape where connecting rod bearing clearances are within specs. But if something a bit loose, the piston may be able to contact the head. So you may have to resort to a head gasket to increase clearance..
this is all speculation until you get down and investigate things..