In fairness, it's going to be a struggle to make any noticeable gains from small mods on that lump. If you were lucky and got, say, 10% from a free-flow exhaust & filter (what I would class as 'easy mods') it's still only 10% of 50bhp (or whatever it makes) so you're looking at a 5bhp gain. Un-noticeable to be frank in my opinion.
I might be wrong though, would be interested in what other folks think.
Personally, either do an engine swap - (even with an SPi 1.3 you will notice the difference straight away - as the Americans say, there's no substitute for cc) or instead or trying to make more power out of the lump, beef the brakes up instead so you can carry the speed you have for longer!