only twin cam parts are swapable between twin cams, ie 7mgte turbo manifold onto a 5mge, or 6mge race headers onto a 7mge etc
single cam parts are not swapable with twin cam
mteu 4me and 5me SOHC bits are generally swapable (i think)
entire heads can be swapped b/w DOHC and SOHC but this requires a LOT of work - timing covers, dizzys, crank perhaps (not sure if the woodruff key is the same accross all models) anyway you get the idea...