You could go with a Razor MX500/MX650 and swap the motor, battery, controller ($500 stock and 15mph). With new powertrain, they can go up to 65mph with $1-2K in lithium and brushless upgrades. Don’t forget to upgrades brakes and suspension. https://razor.com/products/electric-rides/mx650-dirt-rocket/

If you want something bigger, more Surron-ish, and not DIY, go with the Talaria XXX ($3150 45mph). https://lunacycle.com/XXX/

If you’re looking to keep it mini, check the Emove RoadRunner ($1350 35mph) or RoadRunner Pro ($2500 50mph). https://www.voromotors.com/products/emove-roadrunner

Electro and Co. also sell a complete minibike called the “Electric Trail Machine.” Its $3000 and goes 40mph but is only occasionally in stock. https://electroandcompany.com/collections/bikes/products/etm

The EBOX 1.6 or 2.0 are another great option. They go for $1350 (28mph) and $1650 (32mph) respectively. https://eboxelectric.com/ebox-electric-pit-bikes-1-and-2

These are just a few “minibikes” that come to mind. Prices and speeds are approximate. It really depends what terrain you want to ride, what range you need, and how fast you need (or want) to go, and if you’re looking to mod it down the trail. Cheers mate!

Edit: I missed the part you wrote about riding on sand. You’ll want as much power as you can afford and paddle tires.

A guy at the makerspace has one that looks sorta like that but has a wheel motor. His only complaint was that.the read wheel is a little hard to remove.

I just looked through hundreds of images and I can’t find it.