Can I modify a roof truss by removing purlins and kickers?
No, you shouldn't make structural modifications like this without them being approved by a properly licensed and certified structural engineer.  Although you didn't say where this is, most likely it is against the law to do so anyway.  In most jurisdictions, it will require something like a "building permit" to make modifications like this.  This will trigger inspections, which will likely require drawings signed off by the right kind of engineer.