I'm just wondering what are the restrictions on aircraft types that a PPL holder without any additional ratings can fly.
Firstly this guy is in Singapore!
From The Singapore ANO:
To be valid a PPL must contain an aircraft rating:
Class ratings are established for aeroplanes certificated for single-pilot operation and comprise
—
(a) single-engine, land;
(b) single-engine, sea;
(c) multi-engine, land; and
(d) multi-engine, sea.
Type ratings are established for —
(a) each type of aircraft certificated for operation with a minimum crew of at least two pilots;
(b) each type of helicopter certificated for single-pilot operation except where a class rating
has been issued; and
(c) any type of aircraft whenever considered necessary by the chief executive officer.
Presumably you have a single-engine land rating; it would appear that you can fly any single engine aeroplane that is not "type" rated under the Singapore administration. I suggest you look at
Civil Aviation Authority of Singapore