is there a larger or better one to replace the stocker for mine has a good leak.

 

I have not found a direct replacement that's any better than stock. The stock part may not be the most efficient, but it's pretty durable. I don't hear of failures very often.

If you need more cooling, consider adding a second plate style cooler.

 

my stock one has cracked and is loosing about a quart every 2 weeks.is the 3500 or 4500 ones any different.

 

They carry the same part number, so they should be the same.