I'm now able to override the original point of sale ticket/receipt.

In my custom file /Volumes/odoo/addons/report_lapagept/static/src/xml/pos.xml, I had just declare PosTicket again and it override the original.

<?xml version="1.0" encoding="UTF-8"?>

<templates id="template" xml:space="preserve">

    <t t-name="PosTicket">
        <div class="pos-sale-ticket">

....................
            

UPDATE #1

Now to override model.js, we have this in the file /home/odoo-test/odoo-test/addons_pt/pos_lapagept/lapagept_templates.xml  :

        <?xml version="1.0" encoding="utf-8"?>

            <openerp>

                <data>

                    <template id="assets_backend" name="point_of_sale assets" inherit_id="point_of_sale.assets_backend">

                        <xpath expr="//script[contains(@src,'widget_base.js')]" position="replace">

                            <script type="text/javascript" src="/pos_lapagept/static/src/js/widget_base_pt.js"></script>

                        </xpath>

                        <xpath expr="." position="inside">

                            <script type="text/javascript" src="/pos_lapagept/static/src/js/widgets_as.js"></script>

                        </xpath>

                        <xpath expr="//script[contains(@src,'models.js')]" position="replace">

                               <script type="text/javascript" src="/pos_lapagept/static/src/js/models_pt.js"></script>

                        </xpath>

                    </template>

                </data>

        </openerp>

Let me know if you need something else.