As part of the ACE team I clarified EXACTLY what the wording meant re modifying chassis and monococques . You are allowed to strengthen by seam welding on cars you can even add in extra crossmembers. What you can't do is remove any crossmembers , cut ,lengthen shorten, remove anything from the parameters of the chassis
If the same rules apply to bikes then I'd say a hard tail or chopping off anything that doesn't BOLT on lands you in MSVA territory. MSVA doesn't look that hard at all to me anyway ?
Yep the MSVA is easy. If you can't pass one you'd probably be better off getting someone else to do the work for you. It's actually really only concerned with basic common sense engineering principles and mostly EU regs.
There are two issues here. The first one
If you alter the main chassis by cutting and welding it (including removing gussets) you are deemed to have structurally altered the chassis in a negative sense and therefore will need a MSVA. You can remove and lower the seat rails etc and change the swing arm and make it a hard tail by welding on tabs to do a bolt on conversion. The principle being that it could be returned to the stock bike with the use of spanners (those tools not the chap on here). Weld on hard tails are not acceptable.
The other issue!
There is also a points system in place still and you need 8 points to maintain the identity of the vehicle so it's not just about the chassis/frame
Chassis, monocoque bodyshell (body and chassis as one unit) or frame - original or new and unmodified* 5
Suspension (front and back) - original 2
Axles (both) - original 2
Transmission - original 2
Steering assembly - original 2
Engine - original 1
If you don't get the 8 points then you technically need an MSVA.
Saying all of that every inspection I've had has been carried out by someone who has little or no knowledge of the original vehicle and would not have known what was original and what was not. The fact is though if they pass it as ok then you are legal except if they or the insurance company later decide that you failed to declare that these parts are not from the original donor vehicle. Unlikely at the moment but the way things are heading you never know.
Grandfather rights don't exist. If the paperwork V5 notes the changes you are ok if it does not you then need an inspection.
In due course now that the MOT system is computerised it will likely be down to the MOT tester to notify the DVLA when you MOT the vehicle that it has modification that are not standard. You will then be notified that they want to examine the vehicle. Bike testers will know what they are looking at and as VOSA do regular clandestine checks on MOT stations it's going to be harder to find at test centre that is willing to pass what clearly is not a standard bike. Some people have already been caught out!
Also bear in mind to that your insurance will be invalid even if you have notified your insurance company of the mods as the bike is technically a ringer if the DVLA don't know about it!
Hope that helps