R80ST TYRES?
 

I want to use this bike for trail riding, but have to keep standard size wheels and forks. Can anyone suggest suitable tyres and sizes? the standard size are FT. 100/90x19 and rear 120/90x18. Is this rear too wide for an off road tyre? Would it rub against swing arm?would 110/80x18 be ok? I am considering Pireli MT60's or MT90A/T.Thanks.

Both sizes are common in dual-sport tires. The rear will take the same tire as for a R80 G/S of which there are still many made (Metzeler Sahara for instance). The front is more common on modern dual-sport bikes. My search uncovered several in that size. As to the specific tire, look on the past posts or do a search and you will uncover lots of good discussion on the topic. It all depends on what your riding style and objectives are.