Hi. Just my opinion but I think you need a conversion kit that works with a throttle. You may be better off with a lighter ( and more legal in some places) 250w kit as there not be enough clearance for bigger motors (you need to check this) and extra torque arms especially if forks are alloy. (also check this). The front wheel needs to be correctly fitted.
As you have no pedals, I would assume a battery consumption of up to 16-20Wh per mile on level ground and reliable maximum hill climb of 3 to 5%, all depending on riders weight,speed etc. So for example, with a small 9Ah battery, your range could be as low as 16 to 20 miles depending. (rough estimate)
Designed maximum speed would be about 25 km/hr but you often get a little more.
What you would have then is not legally an ebike or pedelec but a (powered) personal transporter.
PS. There is one company that supports and advertises on this website that might be a suitable (throttle only) kit. I fitted one a few years ago and it had a 'soft start' that reduces the 'kick' or pressure on the forks. There are other suppliers.
Cyclotricity electric bike conversion kit with bottle battery, fast free delivery, instructions on bike suitability.
www.e-bikesdirect.co.uk