According to Google Maps, the route seems fairly clear although there seems to be a large complex south of Guyhirn bisecting the old track area.
As someone local to the area allow me to give you an update on the current route.
The old route has been lost in several places.
At the Spalding end a housing development north of the river Welland. A large house across the track bed just south of the river. Part of the old formation now a road.
The A16 has crossed the trackbed and, more recently the new route of the A16 has destroyed the formation around Cowbit. Also a house has been built across the trackbed at the former level crossing. Quite a lot of the formation between here and Guyhirn has been returned to agriculture, the a fair amount of the old formation still exists.
At Guyhirn the former viaduct was demolished years ago and a warehouse type building has been built, which is currently a kids activty centre. A little further south and the bridge at Rings End removed. The bridge at Twenty Foot Drain has also gone.
HMP Whitemoor has also been built across the route though there is space for a new formation.
As an aside a lot of the signal boxes are still in place, though derelict and one or two old signal posts still exist.
In my opinion you could get most of the benefits of reopening by remodelling Werrington Junction with a flyover and other remodellig around Peterborough.
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But although it has long been considered, the upgrade of the Joint line is still a long way off -for example, little progress has been made on the closure of the many farm/occupation crossings, especially between Sleaford and Spalding, which will be necessary if the present speed limit is to be raised.
The joint line upgrade is due to be finished for December 2013, which included modernising the crossings and raising the linespeed to 75 mph.
If you're bored you may want to have a look at...
http://www.networkrail.co.uk/browse.../2010/Route G - East Coast and North East.pdf
There's a few bits about it in there.