sigh.. ok for starters.. if you dont know what your talkin bout..STFU.... PO or sit back and learn from someone who does... dont make assumptions, dont give ya 2c as to what it should have.
ok frame wise.. it SHOULD have the same frame and engine mounts as what any classic Z50J has.. the difference is its got a manual clutch rather than the centrifugal...this is a Ducar 125 in a 197? z50J frame with a 50R tank..
Engine wise..Its 50cc man.. how fast do you want it to go!! its gotta spin LITTLE wheels(8inch) LOTSA times to get to anyware close to 70+kph.. and trust us.. when ya going that quick.. on such a little bike, and a car or worse truck passes you.. wooooaahhh..! lol.. Also dont forget about being able to stop.. Going quick is Harmless.. its the sudden impact that hurts... and the Z has FAR from big/good cooling/well working brakes!!
You may find you are somewhat limited to engine choices as while displacemnt gets bigger, so does the stroke, pushing the head towards the front wheel... or at minimum the mods such as IRK etc, as these will not allow you to run your electrical system AFAIK..
You can get 'joker kits' which are a 50cc stamed barrel bored out to 88cc.. so.. from the outside.. all is legit.. inside.. its not.. this will have no effect on the clutch(except maybe burning them out)
You can look at adding a different engine from a china bike, but again, may be limited as to what will work due to the electrical system. (look at quad bike engines if this is the case as they run electric start and parts may be interchangeable)
crf +1 or std HD forks would be a good upgrade before spending big bucks on the motor, also hunt down a xr75/80 swing arm for a little extra length in the rear.. (im still awaiting the dimensions of the rear of CT90/110 guys-there is a thread)
if you want to run BMX bars, the entire tripple gets changed and crf50 replacement attatched, choose your bars, throw on a 1/4 throttle, mayb a 12clock bar
it WILL want to wheel stand!!
ok if you were to strip down your frame it will look almost identical to this..built in 1979ish...
or this built mid 85..
thats it for now but do some searching around for what your after and decide what order you wish to modify your bike.. from there you can helped with the options available..