I wouldnt go as far as crank failure it just puts extra strain and pressure on the crank which can cause it to start to separate. 21cc's your looking at around 156 PSI at sea level compared to the stock 136 psi. So essentially you would be putting an extra 20 psi on the crank which is not that big of a deal in my eyes. Porting however is going to add about 10 horsepower to the motor to an about 50 hp motor bringing you into the 60's. Thats a great deal of power which is why you may want to get it welded. In the end its up to your opinion of what you think your crank can handle given its hours, wear, and durability.