I have de-rusted more tanks than I can remember and use the old trick of filling the tank with apple cidar vinegar - you can get this in 5L containers for use as a horse supplement. You leave it in for 5 or so days and it absolutely dissolves all the rust although it's best to remove any loose rust beforehand by shaking the tank with stones or nuts inside. Provided you have removed all the rust, its best IMH to then seal with POR-15 or similar to stop any rust returning. POR-15 also has the advantage of sealing any pin holes in the tank. Never had a problem with any of the tanks I've sealed.

I've just had a look at my C15 and B40. The B40 rim is as expected with the tab at the bottom but I vaguely remember having trouble with the new C15 rim because the tab was in the wrong place and so I simply removed it by drilling out the rivets. The tab is only there for making alignment of the headlight easier but you can still align it using the markings on the glass.

This company?:

https://midlands-electroplating.com/

I always do my own zinc plating but only plate nuts, bolts or studs if I can't buy them new or in SS form. Ideally, parts to be plated need to be in good condition and not pitted as a result of corrosion. Zinc plating is relatively easy to do with a kit costing less than £100 but does take a certain amount of experience to get good results. 

Isn't it just a case of measuring the length across the 2 x brackets on the petrol tank and the internal diameter of all the holes. The Sellock roll pins are widely available, for example, on eBay:

https://www.ebay.co.uk/itm/333820464676?var=0&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338268676&toolid=10044&_trkparms=ispr=1&amdata=enc:1FEGbfjuoSeqp-Yr0fMrv9w11&customid=CjwKCAjw3K2XBhAzEiwAmmgrAlczHoLR3gnUgEf3ChWc746W4dell8yk2ezYmhDx4L1aezAMn9PKOBoC77AQAvD_BwE

I've had plenty of problems fitting pattern headlight rims which are not always made correctly. You will probably need to cut back the location tab to make it fit. The bigger problem I have had is that the rims are sometimes very difficult to push on to the headlight shell as the rim size is slightly out. Hopefully, in this case, it is just the tab that needs adjusting?

The Rotax 650 cc engine used in the BMW F650 from about 1993 to 2007 was an updated design of the 1987 Rotax engine I have in one of the bikes I ride. The main improvements were a change from SOHC to DOHC and the replacement of the rubber timing belt by a chain and an increase in CR to 11.5:1. My F650GS was a 2006 twin spark, fuel injected version and what I didn't like about the engine was the harsh sound it made at lower revs although this might well have had more to do with the fuel mapping or the exhaust system. It was definitely not a bike for use in heavy slow traffic much more so than my other Rotax engined bike.

The F650GS was the first single made by BMW since 1947 and was a great sales success. However, in 2011, the G650GS was introduced which was basically the same bike as the F650GS but with the engine now made by Lonchin in China. Unfortunately, the G650GS was not a great success and there were a number of problems with the engine especially its build quality which certainly put me off from buying one.

So the engine used in the new Gold Star has a bit of a chequered ancestry but the success of the new engine will depend on how well it performs on the road which will depend to some extent on gearing but also fuel mapping. I have quite a history with Rotax engines, most of which is not particularly good, but nevertheless I would be interested in a test ride on the new Gold Star to see how it compares.

Off the top of my head, if you align the timing marks on the crankshaft and camshaft pinions, this will position the engine at TDC on the compression stroke. To double check it is on the compression stroke, make sure that in this position the two cam lobes are pointing downwards.

The three things that can give you a heavy clutch are poorly adjusted operating mechanism, poor clutch cable run or clutch springs tightened down too much.

The adjuster in the centre of the clutch needs to be adjusted as per the manual. Once done, the operating lever on the RHS of the engine should roughly form a right-angle with the cable when the clutch lever is pulled in.

In terms of the cable, it needs to be in good condition and needs to be routed so that the radius of any bends is as large as possible. If the cable is too long, this can sometimes result in a less than ideal cable run.

In terms of the clutch springs, you would need to take off the chaincase in order to check how far the spring nuts have been screwed in. The less you tighten the nuts, the lighter will be the clutch lever but they need to be done up as per the manual to avoid clutch slip.

On BSA unit singles, the strength needed to operate the clutch lever can vary a lot depending on how everything has been set up and adjusted.