Mystery aircraft triggers security alert in several countries, crew missing

reduction in weight, removal of unnecessary equipment, re-tuning of the engine to operate efficiently at whatever load-level is being planned, coping with heavy take-off and flight conditions, etc.

it's not uncommon for drug runners to modify planes in such a way.

It depends.

Simplest is to make additional tank(s) and add some fuel pump/valves/pipes. Sure, it costs minus part of payload.

In some cases, of very good optimized designs, enough just to load less payload then mtow, to extend range significantly. For example Mooney m-20 with 3 persons fly farther than with 4 persons.

Hard way, to install engine(s)/prop(s) with better fuel economy, and to optimize aerodynamics with better foil, larger wingspan, v-tail, winglets, laminarization, etc.

And it is not guaranteed to achieve extending range, by just do right enhancements. For example, with An-2 modern upgrades, range not extended, but becomes smaller than on original design.

Some planes have extremely large number of variants, because extremely fortunate initial design accepts lot of additions and them constantly enhance characteristics.

BTW want to accent, usually planes live very long.

For civilian designs, to be more concrete, private or similar to SOHO (~up to 4 people on board, or not similar but close - up to 1 metric tonne weight), 20 years is considered young, and usually they flight fro about 40 years or more.

Planes for corporations and for military/govt live even longer - for them 40-60 years age is normal.

And I talked about ages of materialized planes, many designs are from 1940s or even 1930s, but sure, they are refurbished every 10-20 years, so only frames are really old, but engines and electronics slow upgraded or at least, worn-out parts replaced with new (see next note).

But important note - exists experimental licenses for planes, which accept to use off the shelf components, but experimental license does not accept to transport persons without pilot license. And standard license accept to use only equipment which was approved by regulator, even if this is equipment from 1930s, and it does not accept to use newer equipment from other planes (with very few exceptions).

Sometimes new regulations become problem, because for example in 1930s nobody considered computer-navigators be mounted on planes, so they does not have place for new equipment and does not have power supply, etc.

And as I said, they cannot throw out old equipment and replace with smaller new, but have to make some creative things.

Sometimes regulator and big corporations see that limits are too tight, and working on make some transitional things.

For example, some old engines from 1930-1940s uses lead as lubricant (for valves), and cannot work on unleaded gasoline. FAA, scientists and corporations for more than 20 years work on this, but 100% solution still not created, so for list of old planes still accepted leaded gas.

Eg Apache has only a ~250 mile range, (Gripen and F-16 are 2,000+) so it may not be feasible to intercept with the helicopter.