A 48 Volt 800 Watt motor has the same amount of power as a 36 Volt 800 Watt motor so you may want to consider using a 36 Volt motor so the battery pack size can remain the same unless there is room for an extra battery.
We would start by taking the dimensions of the 800 Watt motor that you want to use and comparing them to the dimensions of the original motor. Then you will see if the new motor will fit where the original motor does or if modifications to the frame will be necessary to install the new motor.
The cost for a 36 Volt 800 Watt conversion should be around $200 if you reuse the existing battery pack or around $300 if you need a new battery pack or want to upgrade to a higher capacity battery pack, and a little less for a 36 Volt 500 Watt conversion. The scooter already has a 350 Watt motor so upgrading to 500 Watts would no be very much of a change.
We do not carry a seat for the Avigo Surge 36V Dirt Rider electric scooter.