Welding your motor mount on is kind of a no-no for racekarts, thats gonna hurt ts potential resale value if you ever sell it, and might even hurt the karts handling/turning ability . The long distance between your motor mounts 3 mounting points might end up acting as a brace and keep the karts frame from flexing like it was designed to.
Racekart frames are designed to flex as you turn corners, this frame flexing unloads weight from the inside-rear tire and transfers that weight diagonally across the frame to the outside front tire. This frame flex helps the kart handle better during high speed cornering in 2 ways:
1- Unloading weight from the inside-rear tire lowers the under-steer effect caused by live axles.
2- And transferring that weight to the outside front tire increases its gripping ability...
There's nothing wrong with building your own motor mount, but I'd suggest you clamp it to the frame at just 2 points, and keep those mounting points fairly narrow, similar to the locations of this motor mount:
A flat motor mount might work out fine for you, but a couple of potential advantages of 15-degree mounts are gaining the additional clearance needed to install racing type air cleaners, and the ability to adjust your right-rear hub/tire further inboard, if needed to improve the karts handling caricaturists. Another potential issue is; most all the tuned headers made for your application are shaped for use with 15-degree motor mounts, and your rear bumper might end up being in the way when using a flat motor mount...
See what I mean, this kart has a 15-degree mount: