J_L_G,  the OS 46LA is a good choice for a general purpose diesel conversion. It works really good as a diesel.
  The Supertigre 40 and 51 works good too. The Super Tiger 45 does quite well also.
The Tower Hobbies 46 too.

The OS 65 LA engine is a good one to convert as will the OS 61FX.
The OS 55 AX and 75AX ought to work nice as well.

Any of the Fox engines work good as diesels. The Eagle 60 and 74's are great as diesels. The Fox 45's make excellent diesels too.
The little Fox 15 is excellent as a diesel. Plus the Fox 25's are nice too.

For really big engines the Super Tigre 2300, 3250, and 4500 engines have been converted to diesels and their owners love them.