My understanding is that there are severe problems in altering bolts on Mausers: potential problems in bending the bot and potential problems in welding a new one on the bolt.
First of all, Paul Mauser himself avoided the whole problem by providng your original rifle with an INTEGRAL bolt body and bolt handle. He just machined the whole unit out of one solid peice of uniform steel. When you really take a look at your integral bolt body and bolt handle, you will realize what an amazing thing this really was. Paul Mauser literally machined away the overwhelming percentage of steel in that original steel billet in a very complicated way so that you got the very best there could ever possibly be on your Mauser rifle. There is no doubt that this was the very best solution to the problem and it makes you realize how expensive these old Mausers really were to produce in the first place.
It is kind of a shame to have to mess up this wonderful peice of machine work, but old Paul just did not make these things compatible with modern scopes, hence our modern dilema with his bolt handles.
The biggest problem with forging the bolt handle anew is that if the bolt is not heat sunk properly or if properly sunk the gunsmith stll takes too long in performing the operation, the bolt will loose its proper heat treatment and this will affect the cocking cams on the rear of the bolt. You will then get a action that cocks too hard and too sticky too be worthy of true Mauser workmanship.
My local gunsmith has said that no one would ever want their bolt forged if they could actually see all the fire and banging that actually has to be employed to bend the bolts on these rifles and I don't doubt him. If the craftsmanship of the gunsmith is not very, very good, I am sure that an awful mess can be produced.
I have also seen Mauser bolts reforged that left the very front surface of the bolt stem unbeveled and this interfered with the cocking action also, making the bolt lift unbeleivably hard. If you look at the front face of a military Mauser bolt handle near the bolt body, you will see a little bevel there and it turns out that this little bevel is important.
Finally, when you get the original bolt bent like you want it to be, it turns out to be a little short and comes up a little too close to the scope to really be ideal, especially if you are used to modern bolt handles that were intended from the beginning to be used with a scope sight, like those on the Winchester Model 70.
The advantage of forging the existing bolt handle is that you still have an integral bolt handle that will never fail, like old Paul Mauser intended in the first place.
The disadvantage of welding on a new handle is that the bolt is no longer integral and these non integral bolts can and do break off. This can be due to poor workmanship or because of unrealized incompatiblities between the old bolt metal and the new bolt metal that affect the strength of the weld.
Finally, a really incompetant welder can mess up the heat treatment and make a general mess of things by this method also, although it may be less likely.
The advantage of the new welded bolt handle actually lies, I think, in the ability to get a new bolt handle that is longer, more swept back and which can give greater clearance for your hand and the scope. It should work well if your gunsmith is good and you are lucky with the metalurgical gamble.
So from my perspective, I would do the following:
1. If you don't need to mess with a Mauser bolt, don't.
2. If you have a staight bolt and want it bent, say for an iron sighted sporter, then forging will preserve the integral nature of the bolt and handle.
3. If making a truely first class sporter, I would consider welding if I wanted a bolt handle that was absolutely best suited for a scoped rifle.
4. I would never trust any of this bolt work to a gunsmith that did not have a well known and highly recommended reputation for this kind of custom bolt action work. There is just too much that can go seriously wrong in this kind of work.
5. As someone else suggested, really competent all around gunsmiths are (perhaps always were) kind of thin on the ground.