They're plenty of people who have done this conversion, I for one. My advise would be to purchase BenT's kit it will include everything you need plus instructions (Minus motor etc.).
viewtopic.php?f=15&t=15009Here's the search area if you'd like to look yourself
search.phpThe hardest part of the conversion will be swapping your oil pick up off your current g13 onto the g16a plus you may need to 'massage' your g13 sump to fit onto the g16a with it's new pick-up
It's a very simple conversion with minor headaches, here are a few items that you will run into
Starter motor body may need grinding down to fit
Clutch cable will need spacing
The motor will sit higher so either spacing of the bonnet or body-lift
Air intake will be on opposite side if you use the g16a carby+intake
I hope this helps with your conversion and if I've forgotten anything please add to it:)
Good luck with your conversion!