Offset racekart frames like yours use seat mounting brackets, they're available from this site:
https://out2win.com/catalog/seat_acc.html
You'll need a pair of their clamp-on seat mounting brackets like the Part# 'RRSSA1' on the rear.
And a pair of their Part# 'RRSSF' seat brackets on the front.
You'll also need their Part# 'STZ0150' Tillett Seat Mount Kit, to bolt the seat to those brackets.
But before ordering, measure your frame O.D. diameter. and the I.D. size of where your front seat brackets install into your frame frame, then call or email that shop to confirm all 4 of these brackets will fit your frames dimensions.
Here's seat installation video with lots more info: