1.

It has been government policy for many years to provide a tricycle for people with ability to handle the controls.

capable

It has been government policy for many years to provide a tricycle for people (who are) capable of handling the controls.

2.

The rising number of accidents prodded the Government into denying that the vehicle was unsafe.

caused

The rising number of accidents caused the Government to deny that the vehicle was unsafe.

3.

This decision is the one currently under review.

reviewed

This decision is the one currently being reviewed (being reviewed currently).

4.

With adequate funds almost any degree of handicap can be met.

providing

Providing there are adequate funds almost any degree of handicap can be met.

Almost any degree of handicap can be met providing there are adequate funds.

5.

The low number of drivers involved does not provide a sufficiently large market for mass production.

enough

The low number of drivers involved does not provide a large enough market for mass production.

The market is not large enough for mass production because of the low number of drivers involved.

Because of the low number of drivers involved the market is not large enough for mass production.

6.

A conventional car with its passenger door hinged at the bottom greatly modifies the problem of access of a handicapped person.

easier

A conventional car with its passenger door hinged at the bottom makes the problem of access much easier for a handicapped person.