Davis Diesel do the Traxxis 3.3 but not the 2.5 as far as I know, but there are other who do conversions also.

http://www.davisdieseldevelopment.com/price_list.html

P.S. Ijust rechecked the above link and both the Traxxis 3.3 and the 2.5 have rods made by the same company so perhaps there will be a commonality of parts since it looks like the rod is common to both and maybe the head is also with the 3.3 simply having the longer stroke but the same bore and bolt pattern. I mean why supply the rod for the 2.5 if you don't supply the head ?