The job of rebuilding the Spitfire destined to hang in Glasgow's Kelvingrove Museum is proving more complicated than was thought when the project began almost two years ago.

For a start, virtually all the thousands of rivets in the fuselage have had to be replaced . . . the originals were made of magnesium set into the aluminium alloy airframe. Natural electrolysis over the past 56 years has gradually rotted them away.

Built in 1944, the Mark XXI aircraft, serial number LA198, is the last remaining Spitfire to have flown with 602 City of Glasgow Squadron, RAuxAF, and is being rebuilt at the Museum of Flight at East Fortune as a lasting monument to the men from the city who served with the squadron.

The museum craftsmen, led by project manager Barry Radcliffe and working in a temperature and humidity-controlled tent, have stripped it down to its basic frame and are literally rebuilding it. But replacing the rivets has been a minor task compared with some of the others. Rebuilding the engine, for instance.

Most Spitfires had Rolls-Royce Merlins, most of them made at Hillington. But the Mark XXI had a Griffon and this particular aeroplane had one of only about 1000 Griffon 61s ever built, now very, very rare. The Griffon 58 is more common, and the East Fortune craftsmen have had to rebuild and re-machine parts from 58s to restore LA198's engine.

Even more complex is the

airframe, the skeleton of the aircraft. Having stripped the

Spitfire down to its bare framework, the craftsmen at East Fortune are beginning to believe that over the years much of the plane has been repaired with parts from other Spitfires.

The port wing, for instance, may have come from a Mark XX, and the fuselage may not be from the same batch of Mark XXIs as the starboard wing.

''The fuselage has now been completely renovated and the surface is ready for repainting, and we have completely restored the port wing, which was in very poor condition,'' said Mr Radcliffe.

''At some stage the plane has stood at a slant outdoors for a long time, probably during the 12 years it was a ''gate guardian'' at the entrance to two RAF

stations. Rain water got into the frame and built up within the port wing.''

Over its years out of service the aeroplane was cannibalised and robbed of all sort of parts, so that in the cockpit there is very little left of the original instrumentation or controls, and finding replacements which the rebuilding team can afford can be extremely difficult.

''The instrument panels in the original Spitfires were made of Bakelite, which is not made any more,'' said Barry Radcliffe. ''We have had to build a replica from metal and have had to find instruments from all sort of sources.''

Luckily there is a thriving barter system among other museums and collectors for Spitfire and other historic aircraft parts, and the museum has been able to obtain all the instruments needed, though some of the prices quoted to them have been huge.

For instance, the undercarriage selector, used by the pilot to raise or lower the wheels on take-off and landing, must be replaced. With Spitfires in flying condition now literally worth millions, the East Fortune team were quoted a price of #3000 for one.

The rebuilding of LA198 is being financed as part of the multimillion-pound refurbishment of Kelvingrove, due to be completed in 2004. The government has given #230,000 to fund the reconstruction of the Spitfire.

One of the reasons for the difficulties in rebuilding an aircraft like LA198 is that Spitfires were built during the war from parts made in hundreds of factories all over Britain which were only brought together for the final stages of assembly. Very few were perfect matches. The result is that a great many of the parts available for the Glasgow plane have to be reworked or machined to make them fit.

There would be no such problem with its great enemy, the Messerschmidt Me109, or the American P51 Mustang, which were mass-produced from identical parts which are all interchangeable.

Surprisingly, finding craftsmen to work on the Spitfire was little problem. A single advertisement brought scores of applications, despite the fact that anyone who worked on the original aeroplanes is now over 70 and more likely over 80.

The three men carrying out the work at East Fortune come from very different backgrounds. Kyle Meldrum was a skilled tinsmith at Prestwick, helping build Jetstream airliners before the factory was closed and he lost his job. James Neil has built and flies his own aeroplane, and Ray Baird is another aircraft enthusiast.