Hi Toad
You'll probebly need to post some more detailed descriptions of the problems to get good advice, but I'll have a go anyway.
Cutting out and lumpy running when hot could be a choke sticking on - if it has a cable operated one, did the cable get trapped or routed wrong when you put the tank back on?
Difficulty getting into neutral when hot - is the idling speed too high/so erratic you have to rev it when it's hot? If not the only other obvious thing I can think of that would make neutral selection different when hot could be oil thats well past it's service life. Other than that, a badly adjusted chain is possible, but generally not only when hot.
Assuming that this has only happened since you changed the tank over, other things to look for might be anything that was connected/disconected/moved when that was done - thinking vacuum line to the fuel tap, dirt blocking the fuel filters, carb with dirt in it blocking pilot jet, or possibly flooding if the float needle has some dirt in it.
Others will doubtless be along shortly with other, probably better ideas. Best of luck with it.
