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You need to scan for codes set, more than likely still logging a crank sensor code.
This is the original type engine for this model WRX, as there are changes in late engines that cause no start?
Is the cam timing correct, have you checked the timing belt?
Have you got a friend with a scan tool that can show engine data?
Need to see cranking rpm in data.
Do you need any further help?
I hope my advice has been of assistance to you.
You say new ECM as well?
Do you have the matching key?
1 tooth out on the cam timing will run but uneven, 2 teeth out on one acm wheel will be rough, 2 teeth out on the crank might not even start.