This thread may give you an idea of how it is constructed underneath. This was a very early 1970s dandy 4 or dart as they are now called, with orange PVC.[You must be registered and logged in to see this link.]
The work was carried out by Pheonix who has done a very good job of rebuilding it and is a member of this forum.
The dandy chassis is a galvanised tubular steel structure with main members running front to back with cross struts spanning between them. Later ones had steel plates welded to them for the suspension units to bolt to. Pheonix had her suspension units welded direct.
The detail has changed a little over the years. I know my 2005 designer had very few cross struts whereas Pheonix has many.
I think it is unlikely that the frame has simply bent, it being much more likely to be rust which will need to be cut out and the steel replaced with new welded in. You should be able to tell by having a good look underneath. Photographs with a flash often show much more than you can see directly.
I'm afraid I don't know much about welding but I would guess any garage or welder could do the actual welding. I had some welding done on a car by a chap who restored old cars as wedding limousines which wasn't expensive. What may add to the cost is if any dismantling is required which may well be necessary. The floor is timber and it is likely this will not permit welding to be carried out with it in place.