Transmission reinforcement

Could I bend the horn with the engine in?

Yes, probably with a floor or other jack while holding the body in place.

When it will be bent do You recommend use a duty transmission strap (CSP) ?

See comments below.

Should I think in use torque bars to prevent torsion in a future?

See comments below.

How strong it will be after a weld repair depends a lot on where the crack is.  The weld itself will be stronger than the base metal, but that base metal may be fatigued over the years.

Any competent welder, especially one experienced in automotive welds, can tell you what needs to be done to repair it safely including welding technique and additional bracing, if needed.  If in doubt about a good local place, ask around about who does welding on commercial trucks - They should know at local auto parts stores.  

A competent welding shop can also advize you on whether you'll need to remove the engine/transaxle before the repair.  The gas line exits the frame on the driver's side of the frame and they should place some flame mats around the transaxle/engine if it's still in there so it should be safe, but they'll know best - If they tell you to remove the engine/transaxle there will be a good reason.

My personal opinion is that the CSP transaxle strap won't help a lot after the repair, but installing a Kafer Brace (often called a Truss Bar) would help a LOT because it is triangulated between the top of the shock absorber towers and the transaxle rear mounting points, resulting in a VERY rugged and stiff support system and supporting the rear ends of the transaxle horns so they can't move.  The price is reasonable, too.  

https://www.cbperformance.com/product-p/4501.htm

Good hunting!

A frame horn crack maybe the beginning of a larger issue, you'll need to inspect the inside of the frame horns. Use a shop pick hammer of a ball peen hammer to tap along the frame horn listing for a change in tone..... a dull sound is not a good sign.  I would also recommend using a simple cell phone bore scope through the large female bungs at the frame horn end where the trans cradle is bolted to. I have a chassis that had a thick paint chip hanging off the bottom of the frame horn the rest of the horn ok but this is what it lead to. Had to fabricate heavy gauge frame horn caps and weld them back in. Also available is a tube frame horn assemble replacement.

I agree , all of the VW frame built cars were built on a  used chassis  with no history if they were from the rust belt or ?

a friend of ours  rebuilt his old 1970s tube dragster only to find out later that the tubes were rotting from the inside and would not pass inspection to run again at the drags.