You don't indicate what part of the country you're in so a particular referal is difficult. The problem with buying a "rebuilt" engine is that unless the seller is very reputable you may get one that was rebuilt on the cheap by a novice and not have good results or length of life from it. I'd recommend finding a shop that is associated with the British Motor Trade Association. http://www.britcar.org/index.html If there isn't one in your area then contact one of the local clubs and see what references they can offer.
Switching engines shouldn't affect the value of your car unless it's a very original example, having a good engine is of more value than having the correct serial number for most MGBs. On a rarer car such as a Twin Cam MGA then that might cause a drop in value.
For me, unless the block has been damaged beyond repair and you can do without the use of the car for a while I would have my existing engine rebuilt by a reputable shop. That way you know exactly what you are getting and can tailor the cam and compression specs to fit your needs if you want to increase the performance.
'73 Midget (V6)
'59 MGA (I6) under construction
"There is a fine line between a hobby and mental illness"