I'm looking to convert a map from 1.3 to source, however have no idea how to do it, all I have is the BSP file, anyone got any way to do it?

Any converstion IMO should be done from scratch. Why, because the brush work will be too sloppy from any file imported via hammer. So you end up having to redo most brush anyway and it's just more work in the end.

With only the BSP, no way you can import it with hammer. You need the original file. I think it's a .map file.

So my suggestion is that you start a new map and reproduce the old work.

I beleive dod_tiger was imported via hammer and togunmav could tell you about how painfull and not FPS friendly this was. At least from what I have heard.

I never map for HL1 so I did not try to import any map, but I know mapper that I have talk to that basically said it just was not wort it.

I only really want to see the map for its sizes and such, ratio's and such... then will begin to start another one copy for Source.

well if a tool permit you to bring a bsp into a .map format then you would be able to import it in hammer. But I don't know if such tool exist on HL1 engine. Hammer need to original mapper file to be able to import it.

BSPTwoMAP

This might help you:
http://articles.thewavelength.net/459/