I'd probably just fit the cover without the Oring initially & measure the end movement then find two shims of a bit less than half the movement & fit one each end. Something like that.

Yep the shims ARE available on partzilla and all those places but because I am in Beaverland the shipping is still as ridiculous as it is on eBay. My dealer orders parts in free of shipping charge, but the shims are on backorder and months out, hence why the parts guy redirected me to a dealer in Ohio that allegedly had them...but didn't.

I ordered some 1mm shims from China, might order some 0.5mm shims as well just for good measure. 3$ USD for a pack of 10 and free shipping lol.... A shim's a shim, right?

I guess there's hardened shims like out of internal motor parts & 'not so hard one'. But wouldn't matter greatly for what ya using it for.

Good point...I'd definitely wait for the proper stuff if it was something more internal in the transmission that is constantly rotating...

You should be fine.

The main issue with the remote gear change linkage, is once the outer cover has any amount of wear, the hole in the crankcase into which the gear change shaft fits, will have worn oval. Fitting a new cover helps with this, but they tend to wear out quite quickly due to wear in the crankcase half.

Anyone with a lathe can easily make a bushed housing which fits into the original covers oil seal housing, supports the shaft much better, and means no need for a new cover. The cover can then be fitted with a grease nipple, and heavy oil used to lubricate (as long as the bushed housing has an oil seal). 

Parts of the mechanism itself are often worn, and while some are still available others are not, so everything needs to be inspected very carefully. Worn selector mechanism tends to make gear change action poor, but I would think dropping into gear by itself, is more likely to be a broken selector roller spring? The gear selector adjustment also needs to be checked, if there are problems related to gear selection. 

Sounds like I might be wasting 60$ installing the new cover then? The shift shaft inside the crankcase half feels pretty loose. I don't understand how all these DT100s are so abused, The worst DT175 I've bought is arguably in better shape than both of the 100s I have. I've got an RT100 engine I might just put in there once the replacement clutch basket comes in. At least the RT100 engine has 20 years less of abuse lol. Suppose I will go and re-assess what to do next.

Oddly enough the new shift cover isn't a whole lot tighter on the shift shaft. Feel like I jumped the gun and spent money I shouldn't have. The silver lining is I'm giving my other DT100 to a friend and the shift cover on that engine has a nice big hole in it...so it might come in handy for him. Time to hop into solidworks and design a housing that can be pressed into the shift shaft cover.

After all that do you think the play was in the inner case just pivoting in the outer cover where the shaft comes through if that had a little extra clearance. I'd be tempted to bronze up the shaft & machine until it has running clearance in the inner case & not worry about the hole being out of round in 'that' case so long as the shaft turns. It's got to be heaps better than how it now sounds. Albeit not the perfect cure but how i'd do it haha.

It's quite possible it's a combination of both the slop in the half case and the cover. what a silly design, can't believe the half case doesn't have a bushing either.

Urban dictionary has an interesting definition of "bronze up" LOL...I wasn't familiar with the term so I looked it up. Do you mean have material built up onto it via welding/brazing? I'm afraid I don't have that kind of capability and it would yet again turn into a sitting project...My dad has a lathe on which I could make the bushed housing Yamfan was talking about, and I suppose if could build up material onto the shaft I could lathe that as well...but it's material build up I'm uncertain I'll be able to do in a timely manner. Trying to decide on path of least resistance here...