If you have AutoHotkey
already installed, here is a script that will turn the spacebar into a left mouse button...
#IfWinActive WGT Golf
...to create the script, copy the above into Notepad and save it, but when you save it, make sure the filename has .ahk at the end and not .txt
To run the script, go to the file you just created and double click it, you should then see an AutoHotkey icon appear in your system tray. The spacebar will only act as left mouse button in the game, not anywhere else. If you want to type a space into the chat box while playing, you'll have to do shift+space.
To stop the script running, press CTRL + X, or right click the system tray icon and choose exit from the menu.
If you want to try the script without installing AutoHotkey, you can download the executable version of the script HERE.
Once downloaded, just double click it to run it. CTRL + X will again stop the script running, as will right clicking the system tray icon and selecting exit from the menu.
My concerns on remapping the spacebar as a mouse button are about button travel. My keyboard spacebar moves a lot further than my mouse button does, surely that's going to mess up your timing? But then again your keyboard may be a better quality than mine with less travel.
Good luck though. : )