it is relatively easy to convert a diesel series engine to a petrol. Different pistons and different head to change the compression ratio and then the manifolds and the petrolly gubbins on top. Some say the diesel camshaft is "better" (I can't remember why) than the petrol one. The diesel crank is a bit beefier and it is probably best to change the flywheel for a lighter one!
that is interesting, I didn't know that although now you have said it it seems to be one of those things you don't understand why you didn't think of it. I guess when you have bought the new head and all the other bits like distributor it may prove more expensive - but better - in the short term than buying a complete second hand petrol engine on ebay. Although there are no guarantees the used engine bought will last. It does mean that one could install a used engine then re-con the diesel as a petrol whilst keeping the car on the road.