OKAY, LISTEN CAREFULLY:

the chainring will literally fold under the pressures of fixed gear riding. the weld will do nothing to help you, i can promise you that.

it's not meant to take the abuse of fixed gear riding. you cannot compare the abuse of bmx to the abuse of fixed gear. the wheel still wants to push you forward when you resist/skid/whatever, which puts pressure on the chainring. since one-piece cranks use a single-bolt system, that bolt takes all the pressure. it developed play because of that pressure, but with the weld there is no way for it to develop play. instead it'll fold.

furthermore, that bike doesn't derserve the $450 you put into it. you should've just taken the money and bought a kilo tt.