If you want something now I suggest go with a 4G.
Bicycle Engines is the only place I know that still has real 4G kits.
If you want to jackshaft now the SickBikeParts 4-stroke shift kit is tops.
Note the kit picture is outdated, Jim has modified the base and the new kit base plays nice with the 142/4G combo.
Figure ~$570 with shipping.
If you are not in a hurry...
I suggest you wait for the brand new system I designed and will be selling in a month or so.
10G KCK long shaft with an HS 142 49cc or HS 144 54cc 4-stroke specially designed for right side output shifting builds.
This is my prototype test of the idea over a year ago for basic function.
Since then working with Don Grube the 4G was re-designed and the new 10G's are literally on a slow boat from China to the US.
If you want to wait you can follow along with development here.
It will be ~$100 less system with less moving parts to maintain.