What is the range of a C172? Maybe 300nm or so, assuming flow rate is the usual semi-unknown factor. This flight is about 1200nm so several fuel stops.
I am sending jxc some trip report URLs which give the general idea, but basically you start with the Jepp 1:500k VFR charts, and then move to the US (1995-1998 last updated) ONC 1:1M charts, and tape the whole lot together on the floor of one's lounge. Get a long piece of wood and draw lines. With VFR, one plans two routes: one wholly OCAS, and one which one will fly if one gets CAS transits. Obviously a decent GPS is a must unless one is a masochist.
One needs to transpose the essential CAS shapes onto the ONC charts (because they don't show CAS) and this can come from either Navbox or from the Jepp IFR charts. It gets pretty messy.
I've been VFR as far as the far end of Crete and managed that with just one ONC chart, covering the bit after Croatia.
Mode C will probably be OK for VFR. South of the Alps, and keeping away from Italian controllers (who tend to not know what
they are doing) nobody really cares much what you are doing...
The Alps can be crossed obviously to the east or to the west. The eastern crossing (from Austria) is more on the route. However a C172 should, if light, be able to go straight over the top from Switzerland, with oxygen.
It's no rocket science but long VFR trips like this need a lot of planning. Unless one is retired, in which case no planning is required
And a good GPS because these are not places to get lost like one can do in the UK.