Differences between the SOHC and DOHC

this is from the Honda web site

1.6 liter, 16-valve, SOHC VTEC™ engine (115-hp HX, 127-hp, LEV-rated EX)
1.6 liter, 16-valve, DOHC VTEC™ engine (160-hp Si)

What are the main differencees between these two engines for a (2000 EX and SI Civic). internally and external can i convert the SOHC to a DOHC?

Thanks