Terrific little bike - reliable and fun to ride - spent just under $1000 on bits and pieces - tyres, shocks, cogs etc - sealed the rusty tank, new jets etc. Starts 1st kick and has never let me down and strangely has made me appreciate my Cota more. The two bikes are very different and both have their strengths - I ride them both regularly.
Oh - parts included a new float bowl - the old one leaked like sieve - correct cable routing and idles perfectly on the stand or anywhere esle.
There is a spacer which fits over the gear box shaft before the sprocket, as well as the circlip, but I guess you could make spacer out of a ground down sprocket retainer plate?
That retaining washer is there to hold the sprocket if you have the screw holes and screws, so you put the sprocket on then slide the retainer and twist it, so the tabs hide in the spline groove, then do up the screws. You can no longer do that, so now just slide the sprocket on and clip on the correct and strongest circlip you can find and if its a bit loose put a rubber O ring behind the sprocket and refit the circlip. and replace the O ring when it gets loose again
also its probably better if you have some loosness on the front sprocket because it is unlikely that your rear sprocket is lined up with the front. Less chain/sprocket/ gear wear.
If the front sprocket on a TLR is allowed to float, this will also mean its will be free to twist to some extent, which will mean more chance of wear on the gear shaft. The proper spacer is still available from Honda, and I would imagine can be obtained from any Honda dealer?
and replace the O ring when it gets loose again 
