is there any disadvantage in welding on a mounting plate keeping the engine in the same direction, just behind the axle instead of in front,
Yes, the disadvantage's would be:
1 - You'd have more weight on the back tires/less on the front = reduced steering ability.
2 - Your motor mount plate and necessary gusseting would brace the frame and mess up how its designed to flex for good handling.
3 - You'd also lower its resale value/potential buyers because people that know racekarts, also know and understand my first and second reasons.
or should I try to modify the seat to the left even more?
Yes that's the correct way.
Some seat struts mount by clamping around the frame, these are very easy to adjust.
But many other racekarts have their seat struts welded in place like my kart below was. I had to bend both of my frames seat struts wider to fit this humongous XXL size race seat. I did not heat up the struts with a torch, I just slid a long pipe over them and pried them over to where needed.
You can see how far the painted strut is bent left past the frame rail in this pic below.
You can see how radically I had to off-set my seat in this pic below. I was installing a huge 406cc. engine that was super wide, this kart still handled/turned awesome like this.
ps: I weigh 235lbs. not quite as heavy as you but pretty close... I wear size 38 pants, this XXL seat was way too wide for me and it didn't hold me in tightly around fast corners. Now I've got a XL seat that fits me perfect. So I had to bend my seat struts back inward for the new seat.
More pics/info on this kart here:-->
http://www.diygokarts.com/vb/showthread.php?t=3664
Does your seat fit you?
It should be very snug, but not painfull.
