wah u wana cnc the 60T? 

if the mongoose use square bb with removable crank, u just need to get a pair of bcd 130 crank.
and longer chain...
hollow bb oso can as long it can fit the mongoose...

i no kaki mod, so kenot answer ur Q, gota wait for sifus.... 

Another option is to get an even smaller cassette combinations. There are cassette that starts from 9 cogs instead of 11. You just need to trawl through the internet. Seems more logical too , rather than changing the front chain ring to 60T which actually looks ridiculous. If you can find the 9 cog, the gear is even higher than a 60 teeth chain ring.

60T/11cogs   = ratio 5.454

53T/9 cogs    = ratio 5.888

55T/9 cogs    = ratio 6.111

58T/9 cogs    = ratio 6.444

I come to realise that many riders wants to mod their bikes without actually understanding the theory behind the mods  Study the facts because sometimes the general public is not always right. Your fellow riders might have done it and even he has bungled the mod and is regretting, he will never admit it. If this is the case, you will be blindly following without even realizing. 

FOR REFERENCE the normal ratio  ,  53T/11 cogs  =  4.818  ( for those not familiar, the ratio here means the number of revolution the rear wheels turns on one turn of the crank )

Hmmm... 60T chainring?  Are you sure?  How big are your legs??

That little bike with the 70T chainring is just crazy.  I don't get it.....  Say you can get it to 30-35kmh, it will be like why?? LOL

Ada lah bang!    Also not expensive lorr. You can get from Amazon, my first google but I didn't check further. It's Shimano Capreo. The price is USD37.54 free shipping in US but don't know about here lah. Here's the link, Mau cepat-cepat beli lorr, stocks not many left. 

http://www.amazon.com/Shimano-CS-HG70-Moun...e/dp/B00STRLUHI
http://i1284.photobucket.com/albums/a566/k...zpsjvtf6y1z.jpg