Why do you have to replace it with a Smartform? It's a well known fact that FI is stuck in the 80s as far as output goes, but it's totally possible to get buy with the SAPScript. What are you trying to achieve with a Smartform that is not possible otherwise?
The Smartforms require different code than SAPScript. Normally when output to a Smartform is required a custom program is created based on a standard one. I haven't heard anyone actually doing an enhancement in the standard program. One of the reasons could be that the same program can process different outputs.