Why don't you just work the f#%K out of your 4k motor?
Stick a nice size cam in, port 'n' polish the head, get it port matched, electronic ignition, get yourself a weber and some extractors and have a ball!!
I've got a worked 4k and i have to say it goes pretty damn hard for a N/A 1300
Wouldn't go as hard as a 4age but it sure smokes a lot of newer,small twin cam cars that are on the road today
If it's important for you to stay a 1300 for class reasons then this alternative would be a whole lot less noodle scratching than trying to put a DOHC head on a single cam poshrod engine :(
Anyway.....thats my two cents
And welcome ;)