Hi mate from personal experience with a DT that was clocked by the feds at 64mph, back when they had discretion, and had out run a RXS100 in the process. You are looking at the wrong things. Yes your carb has a 14mm venturi, other than that it is the same as the model preceeding yours which was 16mm and designed from the outset for a ful, power 60cc engine. The carb size wont make all that much difference at this stage, it may even be better to stay at 14mm as that will generate more torque (thinking of ireland here).
The big bore you are talking about, when i was messing with 50's there were big bores and BIG bores. Some of them were worse than a std engine. I spent about 20quid more (worth about 15 pints at the time) on a Autesa (bad spelling) big bore, this came with a lighter gudgeon pin, chrome rings one being high compression and the piston had considerably bigger ports with an extra small one just under the rings. Does this sound familiar jim??
That ran rings round the cheaper stuff, didn't self destruct either apart from reg needing little ends and head gaskets. That ran with bigger main jet and a higher needle position, the exhaust was a fresco cut open and had the insides cleaned up modified for higher RPM. I had to have a little bit of metal taken out the exhaust port on the barrel to let it rev higher. I even had the reed block machined so i could run one rd350 (non ypvs) reed.
The ig was pretty much as was except for a dt100 coil and running more advance than std and kept in perfect nick, and changing the higher temp plug regular.
As far as gearing goes, 12-36 and 15-45 are exactly the same ratio but they will make your bike run very differently due to the implied ratio between the rear sprocket and the wheel its self, the smaller numbers being better. Don't forget a closer more tarmac pattern on your tyres intead of full on blocks can give you 3-4 mph.
That little lot with a couple of drops of five star could get me to over 70mph in, i have to admit perfect conditions and i must point out it was a complete ba5tard in traffic or town stop start stuff. And i wont mention stopping ahem.
I know i will never know but i hope the co#k that nicked it got his cumuppence VERY painfully.