hi,
if you are driving the 1.8 model then you should look at the mazda FS-8 engine, twin cam 1.8.
If its a 2.0, your option is the mazda FE3 2.0 Dohc engine. These two are plug and play engines for your car and half cuts from similar models are the same.
If you want performance, then you can look at the B8 engine but this requires a lof of modification.
I saw your question on the V6 engine. There are people who have done it but i must say....it is not easy. the best solution is find a plug and play engine. if you want to turbo it for performance i can forward you the contact of someone who has done it.
To get you started, check out these two links >
Reliability of Mazda 626
http://www.zerotohundred.com/newforums/showthread.php?t=30535
Telstar/626 gang
http://www.zerotohundred.com/newforums/showthread.php?t=38925
You can also run a seach for Telstar in this forum.