Hi Adrian,
May I ask what engine you are using?
I would think that cutting the offending diagonal out wouldn,t be too difficult with a grinder/hacksaw then possibly repair the powder coat with the "Plasticote" type of paint ( available at B&Q in rattle cans for about £10 per tin),
I,m no chassis "Designer" but would have thought that if you can put in an extra diagonal between Br1 & BR2 to TR1 & TR2 so that it has ineffect a "V" brace wouldn,t be a problem in my opinion, but would suggest you do the same configuration on both sides to ensure the chassis has the same strength / flex in a uniform way, ( if that makes sense ?
)
cheers
andy