I wish to install a hydronic kick plate heater but it only has 1/2" inlet & outlets the zone has 3/4"pipe should I put in a bypass around the heater to keep the flow the same?

If you are going to keep it on the same zone as other items yes. If you are going to control the kick heat with a separate zone no. But if you put a bypass in you will get very little flow to the kick. You can just tap off the 3/4 with 3/4 x 3/4 x1/2 tees and just splice the kick in to the supply and return lines and add a zone valve to control the kick.

Can I connect the kick with 1/2" tees and if I run a 3/8" bypass around the kick would that create enough back pressure to not need a valve?

can I connect the kick with 1/2" tees and if I run a 3/8" bypass around the kick would that create enough back pressure to not need a valve?
Not really. You need the 3/4 flow to the old system.

Half inch pipe creates about 5 times as much back pressure as a similar length of 3/4'' pipe. However your system could have enough reserve capacity to maintain adequate flow without a bypass on the kick plate. A 3/8'' bypass might not do much good since it would create 16 times as much back pressure as a similar length of 3/4''. If you have an existing 3/4'' line in the area, use it with a ball valve. Ball valves don't throttle quite as well as a globe valve, but seldom leak around the stem. Just fiddle with it until you have enough flow through the kick plate.