You can override the default report like below

1) Create a xml file in your module custom_report.xml with following content

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <report auto="False" id="sale.report_sale_order" model="sale.order" name="sale.order"
                rml="your_module/report/sale_order.rml" string="New Quotation / Order"
                usage="default"/>
    </data>
</openerp>

2) Copy your custom rml file to your_module/report/sale_order.rml

3) Create your custom parser your_module/report/sale_order.py

import time
from sale.report import sale_order
from openerp.report import report_sxw

class order(sale_order.order):
    def __init__(self, cr, uid, name, context):
        super(order, self).__init__(cr, uid, name, context=context)

from netsvc import Service
del Service._services['report.sale.order']

report_sxw.report_sxw('report.sale.order', 'sale.order', 'your_module/report/sale_order.rml', parser=order, header='external')

4) create your_module/report/__init__.py

import sale_order

5)update your __openerp__.py file

'data': ['custom_report.xml',]

6) Restart your server and update your module