what is the biggest engine that i could fairly easily bolt up to my '90 323. i would perfer not to change trany or fwd status.

The biggest engine would be a 2.2litre 12-valve from a 4WD 626. Most powerful would be a 2.0 i-16 from a 626 GT at 165 BHP. You shold be able to get a 2.0 or even a 2.5 V6 in there as well if you were prepared to switch transaxles; these engines are tiny but mighty.

so if i went with the 6 would i also have to change motor mounts and cv halfshafts?

Going with the six would be a lot of work, and only you can decide if it's worth it. There was a 323 that came with the 2.0 litre 6 as fitted to the Xedos 6/Eunos 500, but that was the model after yours. Physically, the 6 will fit in your engine bay for sure and with room to spare (they really are that small) but like you say halfshafts and mounts are going to be a problem. I have a 2.0 litre 6 myself in a Xedos, and it is a great engine. The thing is, the 2.0 i-16 4 from a 626 GT puts out more power than the 2.0 (165BHP as opposed to 143 for a 2.0 V6), and the same power as a 2.5. If you can find a good 2.0 i-16, from an effort point of view it's your best choice. They just aren't as smooth or as punchy low down as the V6.