For a Minarelli type engine:
All the jog pipes will require cutting the header and reshaping it to fit the bike. It takes welding to do this.
Enter..jog pipe… in the eBay search.
Lots of them. It’s around $100.
This will be the cheapest way to go.
It will take reshaping. None are bolt on.
This is one of the reasons I suggested the Phantom engine.
It will take altering the timing some. Another thing not necessary for the Phantom.
With a Minarelli..You will also have to get carb jets.. pilot & main jets, to tune for your area.
I had to also change the carb needle on mine to get it right.
In all likelihood the Phantom would only possibly require moving the needle clip up or down one level..
Not trying to discourage.. just facts of getting a Minarelli engine to run correct.
The Phantom will provide all the power a bicycle can ever use..and probably more.
Once you get a bicycle going 50 mph it could mean trouble for many.
Unless you upgrade things on an ordinary bicycle these engines are over powering it. This all means added costs besides the engine. Safe speed has a price one way or another. Make the bike safe to do it or it’ll exact it out of one’s hide.